My OFF-ROAD POPUP trailer has very large frame beams under the floor which creates a hugh area just under the floor to the bottom of the trailer frame. I only have a freshwater tank under the trailer and here is all this unused space to put something in.
I have been thinking of a false door under one of the cabinets inside and let that open up into a pretty large metal lock-up box of some sort...
I definitely could add additional batteries in these areas with some planning for an easy to use drop down hanger arrangement...
My OFF-ROAD Trailer has 15-inch tires and everything sits up rather high. I have always told others I could mount a hammock under my trailer and take a good snooze haha...
Roy Ken