I also have a new 22FB. I wanted more battery capacity than the Gp24 that came with it. I had two Gp27 Voyagers that I had in a class B that I wanted to use, but there wasn't enough roon in the tongue area for two Gp27 boxes, so I built this one out of 3/4' plywood. It's primed and coated with spray-on truck bed liner. I installed a Blue Sea Battery on/off switch, also off of my B, so I could disconnect all of the parasitic loads when stored. The switch disconnects power to the converter, but the breakaway switch is wired direct so I have that function while towing with the batteries turned off. The switch has to be on when towing with the fridge, but that's OK since the batteries are getting charged from the 7 pin connector. I also encased all of the exposed wiring in loom just because it gives a nice finished look.
My next project is to run a propane line extension to the side of the trailer for a gas grill..