You talk about timing! I just completed this morning my addition to our 2007 Roadtrek 210P, a nice cabinet to replace the third seat. I made it removable so that the seat can be reinstalled. First, I took the seat and back out and removed the small amount of mounting hardware that held them in place.
I consulted the ultimate expert on these matters variously known as DW, She Who Must Be Obeyed and my dear wife. We determined we wanted a couple of drawers as opposed to a cabinet and we wanted a shelf on top of the drawers to place things when we were camped.
I was going to build all this from scratch but found a much easier solution. Home Depot now has an unfinished under cabinet standard unit with two large and one small drawer. They are standard height and depth for cabinets like this except this one is 18 inches wide and most are 24 inches. All standard cabinets are 24 inches deep.
First, I cut off the bottom recess where a baseboard (kick plate) is attached in a kitchen. That lowered the unit about 4 inches. Then, I cut the cabinet on the sides and back to eliminate the smaller door at the top to make the whole thing a more appropriate height. That was another six inches or so. Then, I cut that baby off four more inches front to back. I ended up with a cabinet 18 inches wide, 24 inches high and 20 inches deep.
I cut a top out of birch veneer plywood to match the birch cabinet. Last, I had to make a small top to cover the storage area that was in the entry way just under the front edge of the removed seat. Again, birch plywood works fine.
I finished it out to be pretty close to the other wood in the Roadtrek which is maple in mine. It is not an exact match but is close enough and matches much better than this picture indicates due to the light reflection.
Total cost of this project was less than $150 and a little labor and some Okie engineering. We are very pleased with the result. The same thing could be accomplished for a RT190P except the dimensions might have to be altered a bit. Believe it or not, I did all this was my Ryobi battery powered saw and didn't use my table saw.
The cabinet makers generally want a lot to build something like this from scratch and RT wants a lot plus shipping. This cost me about 20% of the alternatives. You do need a bit of woodworking skill or a friend.