With only 3 speeds, and sitting right on top of the engine bay basically, you are probably just not used to it. Our old '82 southwind with the same motor and tranny would do 80 across the desert pulling a loaded trailer, no problem. I believe it had a 4.56 rear end. On a big block, your cruising RPM's at 55 shouldn't be over 2500. A tach is the next thing you add to that big block to know what she is really doing. Is it really struggling like it's low on power, or is it just that you think the RPM's are too high? Will it go faster than 55? Have you tried 65?
1999 Holiday Rambler Endeavor 38'. Cummins ISB 275, allison 3060.
Toys pulled: 22x8.5 open deck PJ flatbed. 5200# axles. Sand car with ecotec engine. Multiple 1980's ATC's(3 wheelers). We're duners!