Enough blocks, boards or Lego’s to level it up.
A way to change a flat trailer tire.
sewer hose.
Proper extension cord if it will be hooked up.
Black and gray is easy. Gray will be full first in normal use. Keep an eye on sink/shower usage but worst case gray water generally backs up into the shower pan first when slightly overfull. And gray is easy to sneak some out of the tank if you’re parked and get er a bit too full.
If you have a 2 way or 3 way fridge, LP setting is your friend if not plugged in. Fridge on DC will kill a decent pair of batteries in relatively short order especially in the summer.