You have many opportunities on the lift side of your pump for an air leak with the various fittings. That's all it takes to prevent self priming. I'd start by testing the pump with a single lift line in a pail. If it works, there's an air leak. If it doesn't, it's a bad pump.

 

We have no problem with self-priming and agree with @Sams stock van.
That's a whole lot of pipes and joints and far too many opportunities for an air leak, or just a valve cracked open.
Best practice is to have a single feed into the pump with everything pressurized down-stream. If it's going to leak on the pressurized side, you will find it straight away. In my experience, if it's not leaking after you first install, it never will.
I run external water directly into our tank, rather than as a separate parallel feed. It has a vent which we open when filling, so there is no risk of over-pressurizing the tank.
For two tanks, I'd run two pumps.

 

Thirds: pump closest to the tank as possible will solve the problem. I'm sure that seems like a helluva pain to fix - and you might be able to resolve it with a fancier pump - but just fix it right and make the pump right off the tank.

 

Agree with the others...a small air leak will cause the problem. When I was doing my proof of concept, I had some bad connections causing the same problem. As noted by @Sams stock van, temporarily connect the pump directly to a single hose in a pail with an excellent connection, and that should indicate if the pump is working properly. I have zero issues priming from either the tank or a jug of water. Pretty sure I have the same pump as you.

 

I guess I was naively hoping for a suggestion of a better pump. Thanks for the replies. The attached drawing sure looks simpler and makes more sense in dealing with closing off the lower tank. My system which worked fine when truly filled with water but it is still a little confusing to use and if one accidentally lets the top tank run out without closing the isolation valve, then air would get in the system and there would be no city water pressure hose to reprime with. So I really must change to simpler. My shower diverter valve which is to send water back to the lower tank would serve as Neal Carney's way to recirculate some cleaning solution. My intent with the shower diverter valve was also save a smidgeon of water while the temp equilibrates but also a way to send warm water to the low tank. Do most folks use the small Shurflo filter or use a larger filter sold by RV folks attached on the external hose as you fill your tanks? In my case the small screen filter will end up hard to access.

 

I guess I was naively hoping for a suggestion of a better pump. Thanks for the replies. The attached drawing sure looks simpler and makes more sense...

Answering the filter question, we fill using an inline "RV" filter while filling both tanks.

Our main tank has a shut off, then the strainer, then the pump. The main tank then goes accumulator then large filter. Then it feeds the manifold for everything: both showers, sink, water heater.

The secondary tank goes screen, pump. No valve because I don't care if it all drained out.

FWIW, I abandoned our recirculating shower, but there is another pump there as well - and a screen plus two filters. But always pump immediately at the source.

 

I was inclined to think trapped larger air bubbles, in all those short pieces and fittings, was my problem not suction small air leaks. But when I took panel off and freed all the tubes and then gave them a good shake, no help, so yall are probably right. Food coloring in the water and all clear vinyl hoses would help to diagnose. I shall make some big changes according to my sketch. Thanks all for your comments.

 

Our setup is pretty simple: two separate tanks. I considered tying them together; but after using the secondary tank to clean off bikes a couple times, I thought I kinda like having a separate "reserve" tank that requires effort to access. I could have done that with a valve; but this works fine and it's simple and reliable (so far). And that pump works awesome to connect a hose with a sprayer and hose off bikes - without worrying that I'm draining the primary tank.

The bottom tank is filled through the drain on the tank as you surmised. Pressure-filled until the water comes out the vent and it's full. The lower tank output is accessed from a hose bib in the slider step - underneath our galley, which is where the primary tank lives. So to fill the primary from the secondary, we attach a hose and fill. Just did it this week: drained the main tank, transferred to the primary - took a little longer than me taking a leak. 😁

You can see where that's located here - right below the galley. So... easy to run up to the upper fill.

For winter, I will either leave the lower tank empty OR I'll keep the heat-pad turned on - it's kept the tank from freezing even when it's been 30 below freezing at night.

 

Your two photos are really useful to me. My indoor tank, 30 g, and my pump manifold area is in the same location as you. Only I have a bed above and you a galley, so you have more height. My below tank, 16 g, is in the long narrow slot under the sliding door, and with a tank heater. Your floor level outlet must be your upper tank drain and pressure fill? Also there is a clear tubing with the end cut at 45 deg. Is that a vent that you have run up around a gentle U and then back down? While you said you use your gravity fill to move the lower tank water to the indoor tank, is a gravity fill needed for regular water filling?

Our setup is pretty simple: two separate tanks. I considered tying them together; but after using the secondary tank to clean off bikes a couple times, I thought I kinda like having a separate "reserve" tank that requires effort to access. I could have done that with a valve; but this works fine and it's simple and reliable (so far). And that pump works awesome to connect a hose with a sprayer and hose off bikes - without worrying that I'm draining the primary tank.

The bottom tank is filled through the drain on the tank as you surmised. Pressure-filled until the water comes out the vent and it's full. The lower tank output is accessed from a hose bib in the slider step - underneath our galley, which is where the primary tank lives. So to fill the primary from the secondary, we attach a hose and fill. Just did it this week: drained the main tank, transferred to the primary - took a little longer than me taking a leak. 😁

View attachment 196777

You can see where that's located here - right below the galley. So... easy to run up to the upper fill.
View attachment 196778


For winter, I will either leave the lower tank empty OR I'll keep the heat-pad turned on - it's kept the tank from freezing even when it's been 30 below freezing at night.