Reading this from RV site in Ouray, CO. Last night, two families in rental RVs from TX pull in next door. Immediate, big electrical Problems. Amazing park owner was in their RV diagnosing electrical issues until at least 9:30 last night when I went to bed.
-Full set of tools, sockets, etc.
-Drill/bits including square screw driver set
-Multipack of fuses that fit your RV; find all the little obscure ones you didn't even know existed in your trailer and make sure you're covered
-Multimeter
-ViAir RV air compressor (biggest one you can afford, no scrimping on this one, will save your ass)
-While you're at it, get the ViAir attachment kit to blow out your lines/winterize
-Air pressure monitoring system for all trailer tires; monitor your pressure like an obsession
-Progressive Industries surge protector with full protection; note, not just surge protection/cheap model, but the one that actually monitors for incorrect voltage, etc. Will run you $300-500 depending on 30 or 50 amp. Worth every penny and can save you thousands of dollars. You'd be amazed the high dollar RV parks and even camping world where the lines aren't set up correctly and could have fried my electronics
-Spare water pump - these go out at the most inconvenient possible time
-Extended warranty protection from Good Sam or similar. Ask me how I know
-RV Travel Assist from Good Sam or similar
-Roadside assistance specifically for RVs; need a tow? your regular insurance isn't going to be much help here
-Decent set of walkie talkies for parking, etc. with your wife. Phone won't always work. Parking your new big RV Is a recipe for divorce

best be able to communicate clearly and quickly
-Good attitude, patience, and realize that things will break.
-Rhinoflex sewer hose; longer than you think you need. Don't scrimp; get the good orange and black one
-Wilson 4GX cell booster and antennas
-Umbrella insurance policy. Kicking myself here as I haven't done this yet, and it's stupid that I haven't.
-heavy duty Command Strips and industrial grade Velcro roll. We use these all the time. Don't drill into your walls. Hang electronics, jacket hooks, you name it.
-Your water heater Likely has a special fuse that you've never seen before, won't come in any kit, and can't be found in any hardware store. Go look find out what it is (look at model, call Camping World for fuse model, and get a 2 pack)
-Decent headlamp and good flashlights. Guess who's going to be outside fixing some random thing in the middle of the night?...you'll need both hands and good visibility
-Those refillable humidity sucker uppers; put one in bath, closet, elsewhere. Your wife will know.
-Camco water filters that attach to your hose
-Brass water pressure regulator
-Decent set of tire covers (and use them)
I'll post more if I think of anything else.