By Caterpillar RV Performance Guide
The Caterpillar RV Performance Guide indicates that it takes 103 HP at the rear wheels at 55 mph on level ground to move a 34,000 lb coach and 110 HP at the rear wheels at 55 mph to move a 40,000 lb coach.
At 70 mph it is 194 HP for either coach. Climbing hills becomes a different animal entirely.
Climbing a 6% grade at 35 mph in a 34,000 lb coach requires 218 HP at the rear wheels while a 40,000 lb coach requires 266 HP.
Increase the speed up that 6% grade to 55 mph and the 34,000 lb coach now requires 402 HP at the rear wheels and the 40,000 lb coach requires 462 HP.
When using close to the full power available, the 500 HP Cummins will burn over 25 gallons of fuel an hour.
Weight takes power to move and power requires more fuel being dumped into the engine. There has been a good range of numbers thrown out but in the end, it will depend on the speed you drive, the terrain, the wind speed and direction, and how fast you want to climb long grades.
The highest gear with the lowest RPM will usually net the best economy. I would either use the ‘economy mode’ on the selector or add a little RPM to that so the trans doesn’t hunt back and forth between fifth and sixth