Ok since you are thinking 'outside the box' the propane side box that is, then that opens a new path to think on.
I don't know the rules about the installatoion on/in motor vehicles, but the cylinders used in cars and vans for gas power must meet some sort of standards, some on here may know all about that. Even the ones mounted on forklifts must have requirements. That said what can't you mount a larger tank (maybe only one needed for your purpose) on the rear like you are talking about boxes but without the box. That would allow easy fill by a dealer and would not be need to be moved. Worth a thought!
These are the boxes on my old TC.
BoxesThis link shows the new TC with the box set up.
New TC boxes about half way downIf you were to do a tall box standing on you bumper you could double stack the current tanks...and they would be so much closer to a better level to make it easy to move etc. Running the new hose would be easier as you can go into the gen compartment without much fuss. Less cost not having to buy new tanks just a box, but they are sturdy and think about what you can do with the old tank space, six more batteries...really get off the grid and then maybe you wouldn't need propane...HEHE!!! I look forward to seeing your solution.
