I would add
50 to 15 amp adapter (might not need for this trip, but may need sometime)
Assorted tools (hammer, wrenches, pliers, screwdriver)
Rhino sewer line (if you don't have one already)
collapsible ramp thingy for sewer line (totally helps draining, according to the DH)
Small Axe (if you are going to have campfire, we didn't have one on our first trip and it made it hard to do!)
We just spent the last month buying stuff for the RV and I think we FINALLY have gotten done buying. Amazon, Camping World and Harbor Freight were our 3 favorite places last month.
Also, Bed, Bath and Beyond for household stuff.
Good luck and happy camping!!!