since i used a 19 gal residential (110Volt) electric water heater. I knew i would need a expansion tank. I ran into this same situation when i plumbed my house in 01. since i didn't want to use all the space of a round tank i decided to make a "Tank" out of pipe like i did on my house from years ago.
This time I building a 2X4 framed wall that was about 3 feet wide and it dawned on this was the best way to hide my air "tank". I used (3) 2 inch PVC pipes 6 feet tall. Now to make this work better i had to use std household type plumbing fixtures and pipes for everything because the more pressure put in the system the more water i could push out of the tank. (i do have a pressure regulator on my RV toilet).
So how this works is like this Before i leave the house i hook up my hose and fill the system and tank with water. I then turn on the heater. once hot I close the valve where the hose hooks up. (since i live at a higher elevation and only have 40PSI water service ) Then hook up my oiless air compressor and pressurize the system to 80PSI. Close valve again and done.
I can get about 5 gals of cold water, and 10 gal more from the heater tank and there will be hot water for 18 hours after unplugging and warm water even after 24 hours.
since the system has NO leaks it will hold pressure indefinitely or till a pipe freezes.
My RV sits with no water in the system for usually 10 months at a time. For this reason i didn't use push connectors or Pex anywhere. I used CPVC on all pipes except for the 2 inch Tanks pipes.
Anyone with a Bus could use this type of system with a pressurized tank to have water without any electric. It would be easy to mount the "expansion Tank" on the roof. The line from the floor to roof does NOT have to be larger at all. 1/2 inch would work fine.