Welcome to our little corner of the Internet!
I had a 1996 Roadtrek that was built using the Dodge van. While I was generally very pleased with it, and kept it almost 20 years, there were a few things I changed when I built my Transit. Some of the changes were due to the design differences between the Ford and Dodge vans, while others were things I didn't care for with the Roadtrek design.
The feature I liked the most about the Roadtrek was its compact fold out bathroom. The double folding doors opening the bathroom into the center aisle made for a roomy space when needed, and very compact when stowed. The design of the Ford would not allow for a recessed shower pan in the floor, so I ended up building a bathroom similar to many manufactured van campers with a small wet bathroom behind the driver's seat. It would be possible to have a fold out toilet only bathroom in the Transit, but the boss wanted a shower (she was right).
The Roadtrek bed in the rear that made up into a dinette booth was also dropped from my plans. I never used the area as a dinette, and never encountered a Roadtrek owner who did either. Everyone left it as a bed. By using Ikea rails for the bed frame, I have a much larger storage area under the bed compared with Roadtrek's design. I was also able to use a much nicer memory foam mattress compared to Roadtrek's cushion sections.
I also chose to have all the fresh water plumbing inside the van cabinets. Roadtrek installed some water lines under the van, which could be a problem in freezing overnight conditions. The fresh water tank is under the bed, just forward of the rear axle, where Ford recommends heavy cargo be located. I also decided to use a European style cassette toilet instead of a traditional black tank and sewer hose. The Roadtrek had a 12 gallon black tank, while the Transit cassette tank is about 5 gallons. This is actually easier because the cassette tank can be dumped at highway rest areas, BLM pit toilets, etc. Basically, any toilet becomes a dump station.
With modern 12 volt refrigerators using a Secop (formerly Danfoss) compressor, there was no desire to install a 3 way absorption refrigerator like the old Roadtrek used. Since the refrigerator was my primary user of propane, the Transit also has no propane system. Hot water is provided by an electric Kuuma marine water heater that also has a heat exchanger coil. Cooking uses either a microwave, small induction cooktop, or a portable Coleman stove for outside. A portable electric heater is used for the few times I need it. Roadtrek used fiberglass insulation, the Transit has much better 3M Thinsulate from Hein.
Hope this gives you a few ideas...
2016 Transit 350 Cargo, 148" WB, Medium Roof, 3.7L V6, Side & Rear Door Glass