However, fuel consumption stated in mpg is only one factor in the calculation of how polluting any vehicle is in terms of total emissions.
There are two categories of emissions. One includes the unwanted byproducts of inefficient combustion, the things that contribute to air pollution. To minimize that impact, newer is generally better than older, and at any age, maintenance is key.
As newer vehicles have become very “clean” in terms of pollutants, the conversation now has shifted to carbon dioxide, the greenhouse gas that is a normal product of efficient combustion. It is directly related to both quantity and type of fuel burned. Other things equal, diesel uses less fuel per mile driven than gasoline but produces more carbon dioxide per gallon burned. Not quite a wash, but close.
To minimize that impact, you do have to burn less fuel, so choose a fuel-efficient RV, consider the aerodynamic profile, lighten the load, modify your driving habits, and most important, drive fewer miles. It does not necessarily mean sacrificing comfort or curtailing travel, but it might mean taking a slower pace and savoring each place you stop before moving on. It can also mean taking a hard look at how much RV you really need to be comfortable.
Of course there are other environmental impacts to be weighed alongside those from driving an RV, including those related to manufacture (van and conversion) and coach operation (LP vs. solar/electric, for example).
EDIT... After I posted this reply, I was struck by the irony of the ad that appeared directly below it, so I’ll share a screenshot for your amusement. It's also a reminder that pretty much any Class B is a good choice in the bigger RV landscape.
