It is always a dilemma. Your RV is limited in carrying and storage capacity. You have to make decisions of what you will carry and what you will eliminate when something new comes aboard (the "something in, something out" emblazoned over your doorway). The other line over your doorway, "If you haven't used it in a year, it goes" is equally practical, though impossible to live up to.
Which brings me to my "things that I have had for more than a year but which will NOT go" list. I know that someday I will need these "things" when boondocking, which will justify for the time I have carried them, immobile and mostly unused, deep in the bowels of my rig.
- Folding Army entrenching tool (also called a shovel). Folds into a compact shape. Can also be used as a hammer, scoop, scraper, and along with a bucket often required by the forest service (FS) during dry seasons if you are boondocking and plan to build a campfire. Available at outdoor and Army surplus stores.
- Bucket, plastic or canvas collapsible. Also required by FS (see above), for putting out campfires. Myriad other uses: carrying water, collecting kindling, as a step stool (not canvas one), emptying a bucket of gray water--never black water--to dump on a thirsty plant when you accidentally fill your holding tank and are miles from a dump station.
- Hand tree or lopping saw for cutting pesky branches that thwack against the rig in a wind or threaten to take off the air conditioner or roof vent. Also can cut firewood.
- Heavy rubber mat. Mainly for putting under wheel if stuck in soft desert sand or on a muddy forest road. Will lay flat on the bottom of a locker.
- Foil backed reflective radiant barrier. You can buy this in hardware or building materials stores. Use on super hot days on windows getting direct sun to reduce inside temperature. Also to reflect hot direct sun from outside of refrigerator to help in cooling.
- Latex surgeon's gloves (package). There will come a time when there is some yucky job you need to handle and you don't want to touch, such as replacing a dump valve or a toilet, handling anything bloody, fighting zombies.
- Several other useful but often forgotten items like duct tape, bailing wire, plastic wire ties, jumper cables, paper face masks, mouse traps, ant stakes, adequate first aid kit--things that when you need them you need them now and don't have time to search for the nearest store.
You will find that most experienced boondockers, based on personal experiences, will have their own list of emergency items. You hope you will never need yours, but you know you will someday.
[Moved by RD to WP, 3/30/16]