I’m commenting on the assumption that you have a Ford V10 with 6 speed transmission. A vehicle on a tow dolly behind a motorhome presents very little aerodynamic drag. On flat land, if you normally drive easily in D, that will be fine for towing. Only if the tranny does frequent shifting to 5th or 4th, would I shift to 4. If you have a tow/haul mode button, use tow haul all the time as it will greatly aid in engine braking.
As mentioned already, hilly situations may call for “4” if the transmission automatically downshifts often.
I have the V10/6speed in a somewhat lighter motorhome than the FR3 but I find D is fine for most all situations, including towing.
2020 Sunseeker 2440DS on 2019 Ford E-450, Trekker cap, Topaz paint