Well, this has been a long time in preparation, but I finally picked up the "new" 1985 25 ft.
Sovereign in Michigan the first week in July and towed it to Bellingham, WA by way of Yellowstone and the Sawtooths in Idaho. The Suburban (6.0 liter, 3.73 rear) did an excellent job. I towed at 60 mph in 3rd (2500 rpm) for the whole trip. Towing mpg averaged 11.7 mpg, with little variation. On the steeper grades (6% or higher), speed would drop back to about 45 mph at about 1/2 throttle. Otherwise, it maintained 60 mph through rolling terrain with no strain.
The transmission in the 3/4 ton Suburban generally stays in torque converter lockup regardless of load until speed drops below about 45 mph. I found that if I let off on the throttle as speed was falling below 55 mph or so on the steeper grades (and then reapplied throttle), it would go out of lockup and give torque multiplication for improved performance. Applying too much throttle only forces a 3 - 2 downshift which isn't necessary.
By the way, outside temperatures were 95F or higher for the whole trip, so the AC was running continuously as well as the engine cooling fan, so I think the fuel economy was quite good. Maximum transmission temperature was 200F briefly on only 2 hills. Otherwise it ran about 180F.