Some type of Fuel additive - Probably like Power service...available at most truck stops.
Only get your fuel from major chains preferrably big name truck stops.....read No privates. Especially true if you cross reservations.......don't buy fuel there. Could be LSD not ULSD and no telling how much moisture and gunk.
Don't know why you'd need extra fuel. Buy brand new atlas(read - Not 2 or more years old) and watch road signs for fueling places. I drive NM, AZ, CO, UT all the time. Can think of once maybe twice when fuel is over 50+miles away....wouldn't want to carry around sloshing fuel behind me anyways.
Pillow, ibuprofren(CRD seats are painful on long trips.....not a lot of room to move legs around either)
Emergency kit with some tool essentials(screw drivers, adj wrench, hammer

, sockets, allen wrenches, etc)
Fuel filter probably not a bad idea...but with major chains for fuel and PS, you
probably won't have problem.
Hope you got plenty of CD's or Satelite radio.......long, long stretches with maybe one or two choices.......neither very good.
Vegas huh.....housing boom there finally slowed down, but be ready for sticker shock from Florida depending on which part of FL.
Good luck.......plenty of road trips in my life, don't drive tired......Plenty of places(Historic and otherwise just cool) to see on the way that you might never get to see again.....Grand Canyon, Lake Powell, Petrified Forest and any other National Park...each has it's own little history....Route 66, cement teepees(Holbrook, AZ), world largest ball of twine, Wally World, etc.....Have FUN!
WWV