The answer really depends on how you plan to use the coach/generator.
If the generator will be running all the time you drive and/or long periods while drycamping, I agree a diesel is the way to go.
BUT, if you just use the generator occasionally (maybe a hundred to two hundred hours a year) propane will work just fine. Yes, they use a little more fuel, but are half the weight and very problem-free, particularly when compared with their gasoline counterparts. And, you could buy a brand new propane generator for the cost of any major repair on the diesel generator.
We have had propane generator on two coaches over the last 15 years, and they have been virtually trouble-free and absolutely fit our lifestyle.