Around 2007 I sat in on a seminar on big Acert Cat C15 engines in Beaver MHs at a rally in Fla. There were many unhappy Beaver owners who were only getting 5 MPG and much less power than their previous non Acert Beaver MHs. The Cat rep said the C15 need to turn 1450 to 1550 rpms to get the best fuel mileage BUT Monaco/Beaver/Safari put too high a gear ratio axle (turning too many rpms) in them and this was the reason for the poor fuel mileage. He did not like my question of why with a higher axle ratio they were unhappy with the power.:S
OP, I'd be looking for a Monaco Exe with either a Detroit or Cummins engine as it will be the same MH without a Cat engine although I'm happy with the power out of my ISL 400. My Aladdin system is very accurate on MPG and number of gallons of fuel I'll need to fill my tank which I lean the MH to the right so I can get another 15-20 gallons into the tank because of the short fill tubes that are lower than the top of the tank. I'm on the heavy side and have a heavy foot and my MPG over 55K miles is 7.0. I actually did a 100 miles test run at 45 MPH on a lonely WY road and got 10 MPG, but I did not want to take another day to get home so back up to 65+.