I too use the PVC ELECTRICAL JUNCTION boxes with weather tight lids. I like to mount these on the trailer side however to cut down on the direct SUN UVs...
I will mount one on the outside wall aligned up with one on the inside wall and drill out a large two inch hole in the middle of the boxes. Then I can use PVC FLEX lines to route the cables where they need to go. I would always add a large short section of solid PVC tubing glued in place to seal off the large hole between the two PVC Boxes. You can pull the weather tight lids and gain access to the cabling.. Everything installed inside conduit from point A to point B. All of my best DC CABLES for Batteries Installations used this idea.
LOWES has the whole assortment of BOXES with different number of ports for any project you may need.

Working with NAVELEX SYSTEMS in my working years for all sorts of remote installation the BLUE SEA PRODUCTS were proven to be the best choice for high current switches-fuse assemblies-external DC sub panels-terminal blocks etc.. The ANCOR BRAND Marine cables were the cables and ring terminals of choice as well.
Roy Ken