I really don't think getting to 45mph in 60 seconds is real slow.
The first thing to do if you have low power is to change your fuel filters. If you get a bad tank of fuel they may have to be changed more than once.
Do you have a fuel pressure gauge?
Your MH is a 1999 model so you probably have a "CAPS" injection system. Loss of fuel or low pressure will cause major problems if not fixed quickly. Look for a thread called "Repeating CAPS failures-need help" in the Cummins engine forum. There is a lot of information in that post.
2000 Newmar DSDP 3894, Cummins 330, Spartan, MM,
2004 Pilot toad