I would say that anything could have happened to your truck at the dealer. I never let a dealer touch mine if I can help it. I have seen spark plugs left loose, plug wires left off, air filters left out, you name it. Somebody probably poured oil in your coolant by mistake.
My trucks build and usage is very similar to yours.
As for your suspension, I would definitely go coil over front. Fox, King, or Icon. I have the Fox 2.5 set at a 2.25" lift. In back I have All Pro Expo springs and they have proven very tough. I'm just using Bilstien 5100s in the rear and they have been fine. I have Timbren bumps, but rarely hit them
as for gas, I only run premium. You are finding out why.
You really should consider airing down on those long drives offroad. I spent weeks at a time aired down running hundreds of miles of ranch roads in Wyoming. Airing down not only softens your ride, but saves a ton of wear on suspension parts. I have onboard air, so when I hit the highway I air up. When I come back off the highway I use calibrated deflators to bring the air down to 18 psi. They work as I drive, and I just remove them a few miles down the road. I bought them probably 20 years ago and don't remember the brand. IMO, your highway pressure is too high to begin with. I run just 36 PSI in my 8 ply tires running "empty" at about 5,000lbs. I run as high as 45 with the truck loaded and a trailer in tow. Even if you don't air down, running a more reasonable street pressure will help a lot.
