I take a basic tool kit and a voltmeter, small axe, small shovel, bungee cords of varying lengths, and a first-aid kit I put together.
For the fresh water tank I take along a 8oz plastic bottle with household bleach and add a couple teaspons of bleach each time I fill up the tank. It prevents the buildup of bacteria in the hot water tank.
I take 4 boards that are each 1-1/2 inches thick and use them to quickly level the camper by putting them under one or more of the tires. Having 6" of total lift that I can put when and where needed has been helpful at times both in campsites and in the boonies. I only level the camper enough so as to avoid problems with the propane fridge.
I addition to the water thief I have a 6" piece of plastic mounted on a valve so I can shut off the water from the hose at the camper instead of at the hose bib. I take two 25' long white water hoses and one 10' white water hose. More than 90% of the time the 10' is all that is needed and it is more convenient to use and keep out of the way than a 25' hose. When I need the extra length I can couple all three hoses to reach a hose bib 60' away and I have had to do this multiple times. One of the 25' is a fabric fold flat type that is very compact but a pain to use. It is only used when I need 50' of hose.