Battery(s) for the camper
heavy duty jack (OEM truck jack will have a hard time lifting the combined weight if you flat.
Make sure they have given you a stinky slinky and 90* elbow and shown you how to connect it and where the dump valves are.
Have them show you all the fuses, power, shut offs, Drain valves for the water system. how to winterize it. how to operate the jacks for loading and unloading ( have them supervise you doing it more than once)
If pre wired for solar have them show you where all the **** wires and connectors are.