Swiss Safe 2-in-1 First Aid Kit (120 Piece) + Bonus 32-Piece Mini First Aid Kit: Compact, Lightweight for Emergencies at Home, Outdoors, Car, Camping, Workplace, Hiking & Survival
First things first, get a good first aid kit ... seriously.
If your camper weighs over 5000 lbs, you are required to have a weight distribution hitch. Don't mess around with your safety, trailer sway is not something to be taken lightly. Consult with your dealer or manufacturer to see what size hitch you need.
This adapter allows you to plug your 30 amp rv to 50 amp service at campgrounds. You won't always need it, but you may find that the campsite you have chosen only has 50 amp. There is also one for 50 amp rv's to connect to 30 amp service. Find out what you need before purchasing.
This locks down your sewer hose when dumping so you don't get a face full of nasty. Many people use rocks or bricks to support the line but I prefer something that's made for the job, your choice. PROTIP: Once you have hooked up your hose, run your grey water for a few seconds first to make sure there isn't a blockage at the dump station, then run your black, tank flush and grey last to clean your pipe.
High water pressure at campsites can do serious damage to you camper's pluming system. This little gadget will ensure that the pressure is always
Things will break on your camper at the least opportune time, be prepared to hadel plumbing issues with a Pex crimping tool.
Get lots of fuses, you'll need them. Note that some campers may also use inline fuses (the glass tube ones) in some areas so you would want those as well. My camper has one for the slide and is behind where the battery sits so make sure you know what you need before you go.
You're going to want a number of bins in different sizes to keep things organized and ease packing and unpacking. I prefer clear ones so I know what's in them at a glance but that's just personal preference.
A lot of people find stock RV mattresses uncomfortable and replace them right away. Most RV mattresses are not the same length as home mattresses so make sure you know if you have an "RV Queen" size before you buy. Alternatively, many people find that an inexpensive 2" foam mattress topper is all they need and can be cut to size if necessary.