The tow/haul mode is typically used for varying conditions to prevent the transmission from shifting excessively. Use normal mode unless you notice the transmission having trouble maintaining gear selections. I haven't had this feature on a MH but in my chevy van the tow/haul mode simply locks the transmission into an RPM shifting mode but doesn't limit it going into the top OD gear. However, it upshifts at 2100 rpm and downshifts at 1700 rpm so doesn't get into top gear until the speed is over 62 mph. See if yours does something similar.