I have an A/C mounted like that Bonair, on my Cox PUP. No worries about the A/C in that location but you may wish to strap a board or make a nice shield in front to make it immune to road debris.
You could have four uprights and a frame built to hold the battery box above the A/C. In my neck of the woods, the local welding/fab outfit would have it done in an hour and out the door for < $100, maybe < $50. Putting the batteries in the back would be a problem waiting to happen, IMO. Also, may not be convenient for any future solar.
I put a steel box on the tongue of my Starcraft, it keeps everything, including all the trailer brake and lights connections out of the weather.
