RE: Battery Charging
Another vote for jumper cables I have had to jump my batts several times to start my gennie to get things running (ie electric jack just to get hitched up)
I would also suggest getting a solar trickle charger just installed one (and new 6volt batteries) and so far I am very happy with it
RE: Battery Replacement
We built a custom built box basically a steel box (looks mostly like a tool box) with a hinged lid we drilled a few holes in the sides to feed wires through put a hasp on it so we can lock it only thing left to do is put a strap system over the top of the batteries so they can't bounce when we go over bumps (don't want to cause a short on the lid of the box) there are 2x4s in the box so the batteries can't move side to side.
The problem we did encounter was the metal frame for the previous plastic battery boxes and where we wanted to extend it to in order to accomadate our new larger box was right where the trailer brake cables entered into the frame so we couldn't weld there in fear of damaging the other components instead we welded to the previous box frame and extended forward without needing to weld to that section of frame
