If you use an AGM, sealed battery, you can locate it wherever you want, just be sure to use adequately thick wire between the batteries. You want both batteries wired into the system full time "in parallel," so that they are always charging, and always contributing equally. Swapping them in and out of the system is unnecessary.
Many people have modified the tongues of their trailers so that the propane bottles sit further apart, and the generators mount up front. Putting weighty items on the back bumper is generally discouraged, and has the potential to lead to rear end separation.
What kind of rivets are you talking about (solid or pop), and where are they used?