Not having your model trailer, I'll just mention a few things that helped add space, or make existing space more usable.
There is storage under our booth seats, but I had to remove the cushions and the plywood support to get to it. So, in the end cap of the frontward seat, I cut an opening and installed a drop down cabinet door. The laundry bag resides in there now.
The rearward had some obstructions in it that precluded the door access, so I built a long pantry drawer for it.
The cabinet under the lavatory has a lot of volume, but not easy to use all of it until I added a shelf just under the P trap. I also added a second shelf to the medicine cabinet.
Under the kitchen counter there is another roomy cabinet with no shelf. I built a free standing inverted U shaped one not quite as wide as the cabinet. It fits my printer on top, a second shelf for a ream of paper, and space under that for dog food and such. I made it narrower than the cabinet to allow for taller items like cutting boards and cookbooks on the side.
Stackable Sterilte containers with lids add more space to shelves and cabinets, Even if you don't stack them, side by side they can help organize stuff like food.
And I use the larger flatter ones in the cabinets over the bed to store clothes.
Now, I need to figure out how to get to the dead space under the sofa....
2014 Sunstar 31KE
1988 Suncruiser 31 RQ
1968 Travco 210