The '03 to '07 5.9 HPCR Cummins has a 7500/15000 miles oil change schedule.
My '03 2500 Dodge/Cummins 305/555 HO with the NV5600 3.73 axle averages 12-12.5 mpg pulling my 11xxx lb 5er. Highway when empty will average 22-22.5 mpgs, all Pencil figures.
This truck does other duties pulling various size trailers up to 16k lbs so a grocery getter smallblock gasser won't work for me.
The really great part of owning this Cummins for me is money I've saved in fuel costs alone plus much less maintenance costs than any comparable gasser (8.1/v-10 or any BB gasser). My break even for the 3400 dollar Cummins option came at 44-45k miles so at 245k miles its been one great truck.
Other diesel owners have their numbers so one size don't fit all here.
And of course claims from some gas owners how all diesels are so much costlier to operate is always amusing.
The OP's truck pulls a small trailer and no other towing duties and has no other towing issues with his smallblock truck so IMO stick with it.