Royce I pull a 29ft.Mobile scout with a GMC 1/2 ton pickup 2001.The truck gets 20 to 21 mpg on the hwy. and 15 to 16 pulling the scout,It has a 5.3L motor gas.{I don't know what that is in American}My son at Bennetts camping center gave me the speck on eng.trans.and rear. I ordered it per his specks and it pulls great. You could call Jim Rist at 817 573 3665 and tell him what your problem is and see if he can help.I hope this helps. Just tell him Dad sent you.

Donn