I think you and your dealer is somewhat confused. Diesel reduce payload , a lot. It is heavy as hell. If you wanted SRW your option is gas only since it barely meets the capacity requirement of a modern full suite hardside TC.
And diesel's extra 20% power is useless when moving something as light as 4k lb weight. You are not towing 20k lb 5th wheel or something. My Gas F350 DRW/AF865 setup can reach 55mph on interstate with 6th gear. Plenty of reserve power if I switch to lower gear. 5th gear can go to 80mph. and 4th and below just launch my truck forward, although at the cost of extreme mpg reduction.
