The biggest difference now is the added cost of a diesel engine compared to a gas engine. Ten years ago diesel fuel was cheaper than unleaded regular gas, not today. Where I live the difference between gas and diesel is .54 gal. If the average RVer puts 60k miles on their MH and you get 15 mpg with gas and 20 mpg with diesel the savings for diesel is $1650 over those 60k miles, assuming gas and diesel will keep the same price differential. If the price of a diesel engine upgrade is more than $1650 the extra cost may not be worth it.