To answer your question: Look at mounting a 6" or 8" pvc pipe about 5' long from the hitch or even in a hitch cradle. A 6" piece 5' is about 4.8 gallons. My father had an 8' piece mounted under the wing of our old Franklin for the kitchen sink.
But if it was me, I would do the following and use the camper tanks:
Spare tire cable retractor can easily be fixed by rotating 90* so it can be accessed from the wheel well. just have to remove 4 small bolts, rotate and then re-install.
License plate usually isn't really blocked since it can usually still be seen when sitting in a vehicle 3 vehicle lengths behind the truck. My tag has never been real easy to see since even my father's old Franklin, but I've never been stopped by the police except when I pulled out too close in front of one so had to get on the throttle and released so much soot he had a black cloud to drive through.
Air bag valves can easily be relocated. They are usually plumbed to behind the fuel door to give them a little protection.