I agree a diesel would be a poor choice and I have owned two. They need to be driven. Good luck finding a 7.3 for a "song". 7.3 trucks bring a premium especially if they are rust free and excab or crew.
Personally I would keep the truck you have, sounds like you get 8-9 mpg a new v10 with camper you might get 10-11. My 6.0 got 14-15 mpg with my older 10ft 2500 camper and the my 7.3 gets 13-14. Diesels really only pay off if you put a lot of miles on them, do your own maintenance, and keep them for 10 years. Otherwise gas is the way to go IMHO.
You might try to improve the mileage of your current truck. With a better intake and better exhaust you might gain a 1-2 mpg. It's not economically sound but if you want to go farther between fill ups it will help.
I had a 1994 excab with a 5.8 and I still miss that truck. The 5.8 is a little weak, but with a few mods you can liven it up.