Vent covers for the ceiling vents, which IMHO should be standard from the factory. Being able to have them open when it's raining or when driving down the road is a tremendous help.
I bought my motorhome used, so it came with some of the most essential accessories already included. The really important ones that I bought myself (and can think of right at the moment) were a decent #2 square head screwdriver and some towel bars to add to the zero that it came with that were suitable for bath towels.
If you have a proper EMS unit built in, one which will disconnect for low and high voltages, I personally don't see any need to get a redundant one. It's kind of like putting coasters under the saucers under your teacups.