Sounds like you have the bases covered for most items. Long trips are just more of the same as short trips. The biggest thing to think about is actual time in the seat. The Bighorns, for example, are fantastic - but driving is pretty slow. Very long uphills and downhills, with lots of sections of 15mph switchbacks, etc. Easily do-able, but don't plan to be covering 100 miles in an hour and a half. Go on-line, and purchase a copy of "The Mountain Directory-West" (downloadable). It covers all the routes in the western US and notes where there are steep grades.
While you may want to run at 60-65mph when possible to make time, budget an average of 40-45mph for the average per day to account for stops, slow areas, etc.
Yes, slower than when just in a passenger car, but your family, and tires, will be grateful.
1200 miles in two days is very, very ambitious when pulling a trailer! You'll be getting up really early, and arriving really late.
