I have four of those GC15s side by each touching, no issues. Agree 4 sixes is the better choice in the OP if they fit.
You can make a chest- high platform even with the tray to use at home to move the batteries out to. You then have to move the back set out to the tray front where you can get at them.
Only problem might be the 1/2" space not allowing for wires on the posts The load/charging pos or neg has to go on one of the bottom set's posts. That post also has the parallel link wire so it can be higher with crossing bent up wires. Add a fat inverter wire to the stack it gets higher.
Perhaps rig post extensions that clear the caps and allow the wires to stay low. Short copper water pipe banged flat with holes in each end for extensions?
Or is there another compartment that could be made into a battery compartment with a vented box like they do in 5er front cargo bays?
Even if that only held 2 sixes, the two pairs don't have to be that close to work in parallel if there is wire routing.
You could have just 2 sixes for 230AH if you can get by with that and more solar. You could have 2 sixes and one 29 and only have to do extra fuss with the one 29.
