You are incorrect on all counts. Ford gasser gets 7.5 mpg towing, Ford says to change the oil every 7500 miles. So where does the 3000 mile figure come from? Spark plugs at least 100 thousand miles as per Ford. So most RV owners will never change spark plugs.
Diesel fuel also costs .65 - 1.00 more per gallon. No one who wants to keep their diesel motor in great shape would wait 15000 mile between oil changes.
I disagree when you say maintenance on a diesel cost the same as a gasser.
YOU CAN ALSO GET Flying J/ Pilot discount card on a gasser too.