Your 2100 rpm and 70 mph comment reminds me of my 3.55 auto truck with the 12 valve, thats my same problem using my 2nd gen. I wish mine had 4.10 gearing sometimes, its great with a light trailer, but with over 8,000 pounds I really am running too fast or too slow with the 3.55 rear.
If it was my truck I would pull hills in 5th and use more RPM, start the hill in 5th and see what RPM you can maintain. With some power mods I bet you could pull nicely in 5th if you wind it out some. I have a mechanical 12 valve, so its not a direct comparison, but my hard pulls start above 2500 rpm in direct and depending on my load I maintain 25 to 30 psi at 1100 degrees with a bombed truck. I can run 45 psi, but it pulls best around 25 psi and 2500 rpm pulling a heavy and tall trailer. On the flat I run about 10 psi pulling heavy. Wish I had a manual sometimes! Life would be easier when towing, but then I would want an auto when I got back to the daily driver routine.
I can't compare much to a manual beyond the 70 mph comment, although I'm in the exact same place pulling hills in OD lugging or screaming high rpm in direct running slower than traffic. The important thing is to drive by RPM and EGT. So, I found out real quick to keep the RPM up. I pull hills at 60 mph over 2500 rpm when loaded because the engine runs at 1,000 to 1,100 egt and 20/25 pounds of boots on most hills with a modified 12 valve. Both my trucks run best over 2200 rpm pulling. My 3.55 truck likes 70 mph in OD with the 3.55 auto, otherwise I have to drop out of OD. Sounds like your manual is not that different than my auto. But, with that said for towing 5th wheels my 4.10 geared truck is much better at keeping the engine in it's sweet spot, it just stinks when its empty and turning 2300 rpm a lot. Overall, I'm glad the 3rd gen was purchased with 4.10's.
When towing heavy I think of these two trailers. I had just brought the Day Dreamer in from the dealer, it was brand new. It weighed about 16K loaded, my big Toy Hauler was bought a year later and it was a monster weighing around 16K empty and 43' long. I hate to think what it weighed with two motorcycles and a full load of junk inside. I had the 95 set up for the GN toy hauler and the 2003 has the B&W companion 5th wheel hitch set up. I put the 95 on the heavy trailer because of the stronger engine and transmission combo. Both trucks run air bags. The 2003 is all stock except for the Air Dog and with the stock trans I always left the Edge tuner on setting 2 or stock. I ended up leaving the 2003 alone, put my parts money in the 12 valve since that was my daily driver, Mama drove the 03 for years. Now she has a new Honda... time to bomb the 03?.... Maybe
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I've never done it, but if I was towing all the time I would probably try an under drive auxiliary transmission on my 3.55 truck, best of both worlds. My 2nd gen truck runs much stronger than my 3rd gen so I like to pull heavy with it, like 18K. The 4.10 truck pulls 15K nice at 60/65, 3.55 truck pulls best at 70/75 mph light or 60 mph heavy. I usually just slow down and run 55 to 60 in direct drive with my 3.55 truck.
When I had my 4 season heavy wood framed Arctic Fox TT it weighed about 11K at 32' long and the 3.55 truck pulled it nicely. Once I went into heavy 5th wheels with the tall wind blocking fronts I much preferred the 4.10 gearing at least with my old trucks.
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