The V8-496 is popular with RV'ers, not just contractors, so there will always be a buyer out there for an extra-clean vehicle that shows well . . . so don't forget that someone may want to buy it off of you at any given stop (and that might help fund the exact vehicle you want in the future).
To quote myself . . came across this thread on an RV'er board:
Truck Buy Saga
A well-chosen truck (as you have) keeps its value, relatively. Book value on mine -- purchased over five-years ago -- is just $4k less than what I paid for it. IOW, depreciation isn't the monster it might be for some trucks, and any reduction of fuel costs is worthwhile as part of fixed and variable costs.