RV Changes Gears to Quickly and won't stay in Power Band

if your engine rpm stays constant 2000 rpm but your speed drops from 70 mph all the way to 40 it sounds to me like your trans is slipping or the torque converter has a problem, smell check the trans fluid to se if it smells burnt or is a dark brown color, dextron fluid should be red, if it has allison transyn synthetic fluid the color will be more of a gold color. you need to take it to someone that can diag Allison transmissions