No power while driving up hills

[Rhodesia]

I have a 2007 Gulf Stream Sun Voyager model 8368MXG 37 foot she weights 22,000 pounds. My question is I'm having issues while driving up hills like a 6% grade and or a 4% grade. I'm not towing anything. I have to use 3rd gear and sometimes I have to use 2nd gear in order to chug along at about 20 miles an hour. after that I'm on a strait highway and I can do about 80 with no problem but it's the hills that I'm having trouble with. I have a Banks Power system installed so thats working, I have a complete tune up to include the Transmission. She only has 44,000 on a Triton V 10 engine. What I find interesting is that other motorhomes that are 40 footers with a tow vehicle have passed me going up a 6% grade with no problem but yet I'm crawling along at 20 miles an hour sometimes 30. These are gas versions and not diesel motorhomes that pass me. So anyone have any ideals why I might be crawling along up these hills while everyone else is passing me by?