<font color="green"> One spare tire is not enough; and, the best tire on the trailer should be the spare.
Too many people will have only one spare, and a rotten slick one at that.
I carry no less than four spare tires/wheels, and more than that a lot of the time.
When a trailer hits something in the road, both tires will hit it.
If it is something sharp enough to get a tire, it will get both.
Also, if the truck has those odd-ball new size tires, that nobody between here and San Francisco keeps in stock, an extra new tire for the truck is a good idea.
I have also found that a long-corded test-light is worth it's weight in diamonds, many times.
Also handy to cure trailer electrical ills is a 100' outdoor extension cord; this will give you three insulated wires that will reach from the battery, to anywhere on the truck or trailer.
TWO hydraulic bottle jacks, at a minimum.
I had a jack quit working, with no warning; so, ever since, I have always carried TWO; if one quits, maybe the other will still work.
TWO of every wrench necessary to change a truck, or trailer, tire.
I have had sockets split and wrenches break; a dozen spare tires are no use if you can't get the nuts off.
Before leaving, break loose and remove, one at a time, every wheel-nut on both the truck and trailer, coating the threads with anti-sieze, and HAND-TIGHTENING.
Out beside the road, in the dark and rain, is no place to discover that some idiot with an air-gun has over-torqued the wheel-nuts beyond reason.
Also, toss in a length of heavy-wall pipe, that will slip over the wheel-wrenches, just in case you got lazy and ignored the above advice.
At least TWO flashlights with new batteries and extra batteries and bulbs.
It is a new truck; if she catches on fire, get back and let her burn.
Go through the bearings/races and brakes, on the trailer, before the trip.
After servicing the trailer, pull it on an out-to-eat trip, maybe fifty miles from home; this will be far enough to let any HOT bearings show their heads, before you embark on the real journey.
Too many people will drag an old trailer out of the fence-row, kick the tires, pump some grease in those bearing-buddy things, and head out on a thousand mile trip.
Hope this helps. </font>