I responded to your other thread and thought of something else.
It's too soon for an accurate calculation. With an 80 gallon fuel tank and only 1500 miles, the computer likely doesn't have enough data to calculate an accurate Miles to Empty guesstimate.
I can speak to this as my old '11 F150 would never be accurate as we towed our trailer and a couple tanks thereafter. It would usually take running 500+ miles without towing the trailer for the truck to have enough data to calculate an accurate MTE.
I know if your case, this is a motor home; however, if it is just out of the break in period and you haven't done many consistent highway miles, the MTE calculation will read very conservative.
Just my $0.02.
2017 Forest River Surveyor 247BHDS...
...Tugged by a 2016 F250 XLT FX4 SuperCrew, Shortbed 6.2L, 4wd, 3.73...
...Joined with a Husky Centerline TS 800-1200lbs WDH