I also have a class C, and made a small platform to make the box level between the drivers and passengers seats when not in motion. It's basically level with the living room floor and pushed back all the way so the back of the box touched the dog house over the engine. We just set it on the bedroom floor when in motion, I don't think she has ever attempted to use it going down the road, but could if she wanted too.
Construction is simple measure distance between arm rests of drivers and passengers stairs. Then measure the distance from the end of dog house over engine to your living room floor and add a couple inches. You want one end to lay flat on the floor. Cut a piece of plywood to these dimensions. I have a small step down to the cab area so I used 3 legs. Now test fit your piece of plywood and with a small level hold it level with one end resting on the living room floor and measure your legs to the floor of the cab from under side of plywood at the corners closest to the dog house. Cut (2) 2x4 pieces of wood to this length and attach with screws and glue. The third leg goes from the center of the plywood to my little step down into the cab. I attached my corner legs oriented front of rv to back not side to side and added a third piece of 2x4 side to side at the bottom for support and spray painted the whole thing.