Could also be that diesel owners only drive there rigs 2-3 years, trade off, where as a gas owner drives them into the ground after 10-15 yrs. Not that I have not driven a few diesels into the ground after 10 yrs with 200+K miles, body is shot etc......
Gas owners may be more of the non carpet leather pkg option setups too, ie contractor specials, where payload is king, not towing power. Those that want towing power are RV oriented folks, so they want a nicer interior.......
Lots of ways and reasons to look at what is available on dealer lots.
Marty