I’m not sure if its more than what you were looking for but this is what I did and it was really easy and not too expensive. Someone else posted that they did this and I copied it. Its a target 3 box storage at the bottom and a 4 box storage on the right. Then i bought some of the same boards at home depot in the shelving section and made the rest. Hope that helps.

I used a common product called Melamine.  11 1/2 “ wide and 72”“ long.  Its basically a laminate product.  Downside its heavy.  Used 3/4” aluminum angle for support.  Attached supports using 3M vhb tape and 5/8″ pan head screws.  Faced the front with a 1 x 2 clear pine boards.  After 3000 towing miles of the Rouser.  Very pleased.  Looked for pic and can not find.  Runaway in storage.

I ordered two extra shelves from my Runaway dealer, which I installed myself. Runaway prefabricates the shelving. To make the additional cabinetry, I repurposed some particle board leftover from tearing down an old child’s storage bed and found a supplier of charcoal gray felt carpet that was pretty close in shade to what Runaway uses. One of the extra factory made shelves went in the front and one high up on the back wall.