[Big Dave]

Looking at your posts and pictures I assume you never pull fifth-wheel trailers. they'd probably be too much for a 6.5 SRW.

I do have some thoughts.

Forget trying to do much while you are pulling a trailer. They vary a good bit and they dominate your aerodynamics. that's revenue mileage so don't mess with it. But if you're like most guys in your business, you never get backhauls so you really need to cut costs on the return trip.

I'd recommend you make a good aero-lid. It will, by itself improve your empty MPG by 2-4 MPG depending on how fast you drive. I had a very crude aero-lid and it garnered me 3 MPG improvement over an empty bed. there are a number of aero-lids guys on here have built. BamZipPow is working on something that looks a lot like what you want. You definitely need an opening hatch. Lack of hatch is why I got rid of my crude aero-top. Check out the "commercially produced aero lid" listed above. For aero excellence it is the gold standard. Bondo has off-the-charts craftsmanship few of us can begin to approach. Search and read all of Phil Knox' stuff. He has turned over every stone at least twice.

Something to think about: Some sort of replacement for your side mirrors. even your little ones (you ought to see the barn doors on my Ford) comprise at least a quarter of your aero drag. If you built an aerocap like BamZipPow, you could incorporate rear view cameras fair to the air flow. They'd be useless while towing a trailer but running empty or escort, you could fold the mirrors in and rely on the cameras to see behind you.

I used to own a 6.2 diesel. Run, don't walk away from grille blocks. Unless your cooling system is substantially better than mine was your cooling is marginal. It would take a lot of fuel to pay for a cooked engine. Cooling failure is why i still don't have that excellent old truck.

As you said you may want a newer truck (you may get into fifth wheels, too). But if you kept the old truck (I always like those years' Chevys) swapping in a Cummins 4BT and a stick shift would make you hard to beat for MPG. The little Cummins is essential two-thirds of a 12 valve and can be turned up to maybe 175 HP and still be bulletproof. The stick (NV4500 or ZF-6) would buy you an additional 2 MPG. that's a bit of a project but there are scads of guys in your neck of the woods that have done similar conversions.