Re: installing a generator..
I have communications sites with propane vapor gennys. For occasional power failure backup use, they are best. There is no carburetor varnish like stale gasoline causes, and the fuel does not get contaminated by condensation like diesel fuel sitting unused. The one thing to watch out for with a propane vapor genny is the surface area of the liquid in the tanks. The liquid needs to boil into vapor to fuel the genny. When it gets very cold, a single upright tank might not boil fast enough to provide all the vapor flow you need. There's an operational reason the big (250+ water gallon) tanks lay on their side like a mini-submarine - maximum liquid surface area inside. At hilltops where we can't get a tanker in, we use pairs of manifolds each having five 100-pound upright tanks connected in parallel. This gives us more run time plus better cold weather vapor production, due to the combined surface area. If you choose the four 20-pound tanks, plumb them so they can all be on together, at least in the cold.
You can usually find the fuel consumption figures on spec sheets at generator manufacturers' websites. For example, a new 2500 watt Onan propane RV genny uses 0.2 gallons or .09 pounds per hour at idle, 0.4/1.5 at half load, and 0.6 gallon or 2.4 pounds at full throttle. Four 20-pound barbecue tanks would last about 53 hours at 1250 watts, or 25 hours at 2500 watts. By comparison, a new Onan 3200 watt RV diesel draws 0.4 gallons per hour at full throttle. A 40-gallon tank would run it full-out for 100 hours. Larger gennys are less efficient for the same loads. Five 100-pound propane tanks last us 115 to 120 hours on an 11 kW genny powering a 3600 watt load. At that rate, four 20-pounders would last only 18 hours. I think that answers your question??
I would make my choice based on how I planned to use the bus. If I were constantly touring, and running fresh diesel through the tanks, diesel would be the most trouble-free choice. If I were homesteading, moving the bus a couple of times a year at most, I might consider propane in external tanks to be practical. If I was doing minimum-power boondocking with solar recharge plus some occasional generator charging use on stormy days (not what you describe), on-board propane might be sufficent. If i wanted the genny to start every time at -20°F, then I would put up with gasoline plus fuel stabilizer.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.