How about when setting up the MH?
Do you all use the generator to power the slide outs? NO
Most of the things in a RV are 12V powered... lights, slides, controls for refer, water heater, and ACs. Some entry level MH do not have an inverter to power the 120V TV or Microwave. If it has a tag it will have a 2000+ inverter/charger to run everything 120V except the ACs. When boondocking we use our 12V house battery bank to power most things. When we need the microwave I crank the genny to start recharging the battery bank... 2+ hours/day is necessary unless you have plenty of solar (400+ watts) and you are not in the shade or a sun less day.
How is servicing a generator that isn't on slides? (any pro's to non sliding gen.)?
Never had one that was not on a slide plus every 300 hours can be years of use.
Getting fuel at the best price? (Costco?)
GasBuddy saves me $300+ each summer... OK, this summer with lower prices $200. A 120+ gallon fuel tank helps to save $$. Going five block for the Interstate, bypassing FlyJ/Pilot will save you $0.10+/gallon.
Is a 40' too long or a 45"?
40' NO 45' maybe, especially with your driveway situation.
Is a 400 cummings about right for hills?
In a 42' it's plenty 99.9 percent of the time. The 00.1 is above 12,000' where the turbo cannot maintain sea level pressure/power. You will not find 400/425 in a 45' coach... 500 or more since 2002.