So we got a used camper shell for $500 that fits.
We need to replace the bottom gasket trims so figure ~$150 for those. Not too bad considering new is ~$2800. It did not reduce our non-towing mileage. It did increase the mpg from 10.5 to 12 on the same trip to and from the farm. That is respectable.
Straight ROI would be 18,200 miles or ~1.5yrs at 0.357 $/mile savings ($3 premium gas). We visit family A LOT and the closest is 530 mile round trip. Our 2017 Ram is about to hit 80,000 miles.
The ROI doesn't count the day to day value of having strollers, camping chairs, etc that aren't soaking wet or sun bleached from living in the truck bed. Hubby wants to paint it blue. I am thinking of blue vinyl decor pattern that can be copied onto trailer as well.