Not being mechanically minded, I couldn't begin to guess whether the mice problem led to the lack of power when towing, but I can tell you about the HUGE difference we experienced in power on our prior coach ('95 HR Endeavor, 36ft, Ford 460) with a Banks System. It would climb mountains without the least effort. Even climbed the 25% (Yes 25%) grade to the Amicalola Falls GA State Park campground (Great place!) without hessitation. We were beside a similiar HR without a Banks going up a grade one day and left him in the dust. I've read that while Banks systems are usually a good investment for adding power to most engines, the Ford 460 seems to get the most benefit.
Don't have any connection to Banks or any installers (ours came with the coach which we bought used), just know we were very happy with the power in our '95 HR, and that was a heavy coach for a gasser.
If you don't find another solution, you might want to look into getting a Banks. Hope this is helpful.
Lois (& John)
'02 Alpine, 38', 400ISL Cummins
'02 Ranger 4x4 toad