That's quite a load of wood you got there ! I used to drive from my home in N. Texas to a vacation home I have in southwest Colorado a couple of times a year (it's now my primary house) and the last couple of years I started loading my '95 Ram up with firewood that I would use in the winter up in Colorado. The load of firewood (8' bed and it would be loaded to about 6 to 8" inches above the bed with wood) had no effect at all on my mileage for the trip.
The wood would cost me nothing since I would gather after spring thunderstorms or simply watch Craigslist and go get it free,,,,just had to load it ! We have a lot of hardwood trees in N. Texas versus mostly pine and aspen in Colorado so picking the wood up was worth the effort for the trip, the oak burns hotter and longer than the stuff that's easily available in Colorado and of course, there was no mileage penalty with the truck loaded. The truck actually rides better with the weight and if there was snow and ice on the highways, you had the benefit of better traction. The free wood essentially more than paid for the trip ! A win win situation, with the added benefit of a little exercise loading and unloading which I don't mind at all. Actually, I like working outside like that and refuse to go the gym when I can incorporate outside physical activity into my life like that anyhow. These are great trucks with an enormous ability to get the job done as you know !!