I installed the batteries and wiring behind the black panel that is next to the removable black panel that hides the pump and other wiring. I cut a piece of 3/4" plywood to fit next to the white panel that the pump is mounted on and screwed it to the back wall. I then mounted two busbars (red and black) that I used to connect the batteries to. These technically aren't needed, if you only use 2 or 3 batteries and you wire them in parallel. I originally thought that I'd be installing at least 4 batteries (2 in the pass through and 2 under the bed). So far I've only installed the two, because the solar panel works way better than I expected. I removed this set up from our old Imagine trailer which didn't have a very big solar panel. I also mounted the Victron shunt in between on the negative (black) battery busbar and the negative "load" busbar. I tried labeling one the pictures to make it clearer. In the pics you can see the Red White wire and the thinner Red wire both running along the top of the board and connecting to black fuse panel, these wires were originally wired to the old battery up front. In the picture showing the power board, the fuse panel doesn't have any fuses installed in it yet. If I were to install an inverter or some other device needing 12V, I would connect it to the fuse panel (fusing it accordingly) and ground busbar. The Red/white wire runs over to copper bus bar that Alliance installed. The thinner red one connects to solar controller. I also reused the original black ground wire that connected the negative battery terminal to the frame. I connected it to the Negative Load Bar and bolted the other end to an exposed piece of the steel trailer frame, so the new batteries were grounded. I also made a picture frame using some wood and a sheet of plastic to protect something from damaging the batteries and wires.
Lastly, I apologize for the delay in responding, but we were on a 5 week trip through Quebec, Gaspe Peninsula, PEI and Nova Scotia. Unfortunately, our trailer is all closed up for the winter so i couldn't take better pictures. If you have any questions, just ask. The batteries work just fine.
I hope these pics help.