Air resistance is the primary fuel user and that's exponential with speed. Meaning that if you double the speed the resistance increases by a factor of 4. Air resistance is twice at 70mph than it was at 50. But no one wants to (nor should) drive 50 on the interstates.
Gonna depend on your truck. I find 65 pretty comfortable with my Ford 3.5 EcoBoost engine. Cruise control in the "slow lane." Good therapy to drive this slow for my Type-A personality too.
New camper (maybe your Surveyor is an RV, my Roo is a merely camper
) needs the lug nuts torqued several times until they stop turning. They're probably loose now -- really. Mine are 90 lb-ft on steel wheels. Find out what yours should be and tighten them today. I'll bet half of them are loose (under spec). Check them every rest area on the interstate until they stop turning. Then every other rest area. Finally just check the torque every morning when you check cold tire pressure (which is the Max cold pressure as listed on the sidewall -- no less).
-- Chuck