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You already have the smallest truck you should be pulling 10K lbs with!
For a gasser, you have a pretty good truck to pull 10k lbs! The only thing better on a gasser with that load in the mountains would be to have 4.30:1 gearing.
I pull a 34' Limited listed as 8300 lbs dry and 9800 GVWR (you might want to recheck your weights) with a 7.3L Ford PSD, 4-speed auto, 3.73:1 gearing and LT265/75R16 tires (same diameter as LT235/85R16). Without the trailer, this translates to 20 mpg at 2000 rpm at 70 mph in OD.
Out of OD this becomes 2850 rpm at 70 down to just over 2000 at 50 mph... which would translate to 3150 rpm at 70 mph with 4.10 gearing. I sure wouldn't want just a big-block gas motor with 3.73:1 gearing... that ratio puts the rpms down in a diesel's power band, but a gasser needs to run 3,000-4,500 to be in its powerband, and pretty close to 3,000 rpm to even be near it.
A HD or Superduty truck weighs a lot more than a 1/2 ton... but that's what you need to pull a heavy trailer. A small-block in one of these heavier trucks can barely pull the truck up a mountain, much less have a heavy load behind it.
As I said, you have the right truck for pulling your trailer. If you want a truck that'll do both the towing AND get good mileage on the highway at 70 mph, then a newer truck with OD tranny and a diesel engine is the answer. But you'll never even come close to any fuel savings breaking even with the higher price for a diesel engine (about $2500 on a used '96 truck, for example) and the more expensive oil changes.
If it were me, I'd spend a lot less on an older econo-car and commute with that.
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