Think the bottle jack was already mentioned, they're pretty useful along with a board or three for under/over it.
I'll also second the air pump idea. I put onboard air on my truck and it's been invaluable. Decent air gauge to check tire pressures in the morning. Or I think you can add extra sensors to the fancy new trucks these days for your trailer tires.
Make sure you have the tools and proper bit/end to lower the spare on your pickup. Turns out my truck was missing the special security bit end when I bought it used and no one had one. Cost me an extra day and had the tire shop cut the spare off when I needed it after an accident in AL last year. Same if you have locking lugnuts, make sure you have that tool to get them off.
Tool kit? If I'm hauling the RV, I tend to have a full set of most of my tools, if another trailer I bring a smaller selection but am sure to have sockets, ratchets, wrenches, screw drivers and bits, etc...
Fuel additives? I regularly use Optilube performance additive and their cetane additive. I pour that into 8/4/2oz bottles (nalgene) I picked up off of Amazon ahead of time. When I fill up, just select the appropriate sized bottles and dump them in.
Electronics - radar detector, phone chargers/cables, spares.
Fluids - oil, washer fluid (top off before leaving and bring some extra, bugs across the Dakotas are crazy at times), coolant? Extra DEF if you need that. I tend to have a tub in the bed of my truck with these.
Filters - spare fuel filter in case you pick up a bad tank somewhere? Never happened to me, but I have one in the tub of fluids and spares.
Paperwork - current registration and insurance cards - truck, trailer, and boat. Health insurance card for yourself.
Added on edit - wife is usually my passenger. She likes to have a pillow and blanket with her. And lots of snacks and drinks.
Oh and a fire extinguisher, somewhere easily accessible.