The motor is probably a MB926 - 330 HP, 1000FP Torque. We have a 2008 390qs with that engine. Get approximately 9 MPG except when driving in mountains - drops to 7-8. We have had a couple of issues with breaks in injector lines. Have had to have two #4 and #5 replaced. The motor runs out well, and does everything we have asked. Freightliner told us the motor likes to be worked - going up mountains 2nd gear and 22-2500 RPMs works well. Oh, my normal service man says if I don't want to return to him for an oil change he would NOT be unhappy. all of the oil and fuel filters are in cartridges (not spin-ons) and something more than half must be accessed through the bedroom floor as they are on the top of the motor. If you have other questions would be happy to try to answer.
Kent & Peggy Savage
2008 Forest River Berkshire 390QS
2008 Chevy Colorado 4X4 Crew Cab Toad
Callie - Fur Kid