The 3rd battery is inside the van in one of the benches we use for a bed and a portion of the dinette. The battery is a ventless Duracell brand. I don't know what the amps are. The wiring runs from my electrical converter into the side of the van and then to the back of the van. There are holes in or near the rear door posts that go thru the floor. To hold the wiring when it is not in use, I added another female 7 prong plug under the bumper.
I had not heard of anyone doing what you are doing and it interested me. A quick search on trailer harness showed me an implementation (random) that gave two 5a circuits for running lights on a trailer. If the transit is similar and you have, say 10 amps to the battery, it would be good for you to find out the 20 Hr AH (amp hour) rating of your battery, measure the loads you want to run and also the actual amperage of that connection. Also, read the user manual for that battery for their charging recommendations.
Most batteries need a minimum of 10% of the AH rating for charging. A lower rate of charge works well for maintaining a charge. Many sealed batteries have a higher minimum requirement, perhaps 0.2 or 0.3C where C is the AH rating. If your amps at the hitch is 10 amps, then the biggest battery that can be supported long term is 100AH, but if the battery wants more amps it could be 50AH or lower.
The reason I say "long term" is that if you are discharging your battery and not fully charging it to 100% it will start to degrade and have a shortened life.
On the other hand, if the loads are relatively light and the battery is inexpensive, the convenience of what you have implemented may outweigh the possible need to replace the battery prematurely, To mitigate this possibility, if you are frequently on shore power, you could get a three-stage charger and fix the battery when docked.
