Back in the day, been maxed out at 25 mph, 1st gear, foot to the floor towing up the grades on the West Coast.. This was in a brand new 1989 Ford E250 van with a 351 in it towing a #6000 cargo trailer for construction work and going between jobs..
I pass you Class C's and A's all the time now with the rig in my sig... :) But, I'm a lot less weight, so it's all relative..
I think it's normal, so just gotta adjust your expectations more than anything else... If it was fuel or exhaust related, you would not be able to pull 4000 rpms and still be going 45 mph... You'd be at a max of around 3000 rpms and maybe 25 mph if it was one of those being your problem (and the engine would be missing or bogging like heck)... IMO... ;)
Good luck!
Mitch
