By law, no removable propane cylinder is supposed to be filled while attached to the vehicle. If I remember correctly, its along the same lines as to why you're not supposed to fill your portable gas can while its sitting on your truck's bed or tailgate.
Only ASME permanent mount tanks are supposed to be filled in the installed position.
Now, my 20lb horizontals are mounted in the overhang area, underneath the dinette (will be higher up in the rebuilt side entry version). So, basically, you'll need to find a cabinet or other space that won't consume valuable interior space to accommodate a new sealed box and then modify the camper framing to allow for it's attachment and the new door.
In my rebuild, the propane compartment is going in the bottom of a cabinet right by the entry door.
To get an idea of how it will look, look at my signature and then visualize that hatch in the picture being just above the tail end of the truck bed.
Also, another option would be to get a frame mounted ASME tank installed forward on your truck's frame and have a quick disconnect and pipe installed so that you can run the camper off the tank on the truck vs the onboard cylinders.
