I would advise that you stick with duals, 410 rear & long bed. I had 2 3500 Dodges (the same except the rearends). We made the same trip to Califorina within year and the fuel milage was within 1/2MPG. What makes a big difference is if you can keep you foot out of it. Drive like you have a rotten egg under the pedal. I find staying under 60 helps a lot.
To save costs, there are areas that there is a lot dry camping available (especially in the SW). If you have a solar panel and gen set you don't give up much in comforts. You might want to concider a MH so you can tow a small car for exploring.
Enjoy your visit.
'01 3500 Ram QC HO 6sp. BD Exhaust Brake