well if my memory serves me correct, you just got a trailer, so added to the list would be a spare tire to teh trailer(or even 2 pending how far you travel, and or even more so on a double axle trailer, as if you run something over on one side, you tend to kill two tires in a row)
next a 12 volt or a good impact powered impact wrench, trailer studs just plain suck to have to chage on the side of the road, many 4 way lug wrenches many times do not have size you need, or the trailer is too low with a flat and you cannot spin them,only do half turns
and if the lugs/studs go thru to other side, and and all of the stud comes out9not like a car truck one, that treads say on) they rust up like mad and fight you till the very end
been there and learned this the hard way , so this is a tool that stays/lives in ever dam turck I own ever since! LOl
a battery charger is a plus too, as many folks have been known to leave a key on or?? and then atv battery is dead whole trip, or if you sink one, get wires wet and have to crank the crap out of one to get going, there nice to have, and take up little space
a few flares and a good working light/flash light is a great idea too
I towed my atv over a thousand miles a few times a yr for 15+ yrs, so, I know these come in handy!
I am also a big fan of carrying in a tool box an assortment of nuts and bolts, alont with tools, can of WD 40, carb/breake cleaner, QT or two of oil, for both atv and truck even!,
in colder area's a blanket is a good idea, can come in handy to keep you warm, or to lay on if you need to fix soming while being on the ground, same can be a good thick piece of used carpet too, can park ATv on it while being towed , or in the bed of a pick up ,too!
again for trailer and even truck, a good jack, can be a life saver, as many factory tools in cars and trucks, just plain suck, or very hard to get to!
a tire plug kit, on atv and a small compressor went on almost evert trail ride I been on, got a small one at wally world that fits in my little storage compartment on my 400 foreman, so it lives in there
I like a folding saw on atv too, a lighter, a small LED flashlight, extra spark plug, and a wrench that goes on easier than the factory one!
some quick zip ties in all sorts of sizes, and two ratchet straps, think mine are 12 or 15 ft one's
they can be used at tow straps too if need be, I buy good straps rated at working loads of 1500 lbs too,, them cheap one's I have seen break way too many times
a few extra fuses for my fuse box too in a zip lock bag
and leather work gloves
and if on longer far off the beaten path rides,a good first aid kit/ with things to clean eyes, like fresh water
as time goes by, you tend to find out what you can do without or need to add
I'm more like a boy scout when I go places, live by the ruel rather have than not need than need and not have
so I over pack more than under pack, but its saved a lot of trips over the yrs having it at times!