We just got back from 8000 miles (I don't know how) towing a C-25, to around and from the Great Lakes. We used Judy's truck: a 2006 Ford
F-250 with a V-8 5.4 engine and a 5-speed auto, 3.73 rear axle. Averaged 8 mpg towing.
I know the C-25 isn't a Tomcat, but it isn't light either. We came over Rabbit Ears Pass (11,000 ft.) on US-40, in Colorado. 2nd gear up, 1st gear down (damn RVs.) On the interstates, cruised at 65-70.
My point is this. 95% of the time one is cruising on fairly level roads. For hills, the other 5%, use the transmission, that's what it's for. One can buy the diesel, or the V-10, or the 454 so that you can go up hills faster, but that's only a short distance, and the rest of the time one doesn't need it. Those bigger engines are expensive to buy and run, exclusive of fuel.
ANY engine of 5.4 liters (330 ci,) will do the job. What one needs is a good rear axles (3/4 ton and above,) good suspension and rated towing capacity (11,000 #.) And a good auto. I know the DORF 5 speed has given trouble, but they did redesign it in 2005, and ours has worked well. I understand the Chevvie has a 6 speed this year.
Boris