A clear sleeve for the black tank connection. It is helpful to be able to see when the rinse water is running clear and most of the solids are out of the tank. Your unit may not have a tank rinse connection so just run a couple of buckets of water into the toilet when you dump.
The X-chocks are a good addition too. When I have to level up and am on the blocks regular chocks don't work so well. The X-chock goes between the tires(tandem axle) and will work in place of a regular chock. Use a regular chock if you can too though.
A small heater like the Pelonis ceramic disk is good to have on hand for a chilly night. Its small and safe with no exposed elements to burn little ones hands or start any fires.
A basic set of hand tools and volt meter is nice to have but don't go crazy with tools. There is only so much you can do or WANT to do when camped. A battery drill for stablizers is nice too. Don't use an impact drill for the stabilizers though, too noisey and unnecessary for that amount of torque.