Thought I'd post some pictures of the shelves we built last weekend in our Trillium
1300. A pretty standard setup of PVC framing and MDF board which we coated a few times with some poly. For the big closet, I ran insulation foam around the edges to help eliminate any shifting and to keep the shelves from digging into the camper's insulation. I also ran a U shape support off the side, under the window to help further support the shelving frame. It doubles as a towel rack! Our propane
grill and heater go on the floor, blankets, pillows and dry food up top. FYI...Lowes sells the bungee cords with the 1" hooks on the end which worked out perfectly other than having to tie a knot in each one to shorten them a bit.
For the smaller closet, which will be for clothes, we went with smaller PVC (I think it was 3/4" and no insulation around the shelves. Still fits snug though, no shifting.