1800mile Trip with 175K mile truck and trailer. Advice?

Weights vs ratings is your responsibility, not the 'weight police'

Tire PSI gauge and an IR heat gun. Check *ALL* the tires at each stop and just
before taking off

This includes the spare(s)

As noted, and just a reminder since you say you've experience in
snow....chains

Chains

Manual hand pump...I carry a 12VDC pump with a loooooooooong hose to reach everything

Engine oil...enough for one complete change (I carry 10 quarts)....do change
that unless you've just done that

Ditto diff & tranny...flush it if you haven't recently

Ditto power steering/brake pump fluid

Coolant, pre-mixed. I carry 2 gallons

Check/change the serpentine belt



Oh heck, check out this thread on what is in your toolbox...

What's in your toolbox