Decided to make the trip down and re-measure.
For anyone else with a 2018 3721RL the tray I decided on was 26” x 90”. It looks like a good fit with some extra space. I probably could have pushed it another inch or two, but after having to return the first one that weigh a thousand pounds, I decided to play it safe.
2018 Montana 3721RL
2018 Ford F350 Diesel
Life Comes Down to a Simple Choice...Get Busy Living or Get Busy Dying...