Need a bit of 'educating'...

While not specific to your MH I find tow mode most beneficial in my truck towing the TT when going down grades. I will use it not towing going down grades too, saves the brakes. I don’t notice any real benefit going uphill.

This is off Fords site.

The tow/haul mode reduces shift cycles. Depending on the truck, it may also boost torque and engage an engine braking mode to help drivers maintain control while driving up and down hills. In general, the tow/haul mode changes shift points to higher RPM limits. This helps you keep moving as you haul your trailer.

If on flat ground you should get better mileage with it off.

I use it all the time as it really saves the brakes. You can go without when driving at slower speeds if you're confident in your brakes. It doesn't prevent using higher gears. The 5 Star Tune is great for the older 4 and 5 speed transmissions. Reviews when installed on 6 speed transmissions don't seem to be as conclusive, but worth looking at. But, when cruising at highway speeds, the tow haul mode is a definite safety factor and I suggest using it.

A motor home is always “hauling”. Nothing wrong with using it all of the time.

When towing our Motorcycle trailer I always just leave it in Tow mode. Wife reminds me to do that. 1500 miles up 1500 miles back.

Tow haul changes the transmission shift patterns slightly to help when the coach is heavily loaded or when ever you want to use it towing or not. It does not restrict a gear such as OD on/off. When in Tow/Haul and accelerating moderately hard, such as a freeway on ramp, the transmission will hold a gear a bit longer before upshifting to get every bit of power available to get you up to speed as quickly as possible. Floor the accelerator and you'll probably get the same effect. The cost is a bit more engine noise with the higher RPM, more fuel consumed and maybe a slightly higher shock when the shift does occur.

Slowing down is when you'll really feel the change. The transmission will get a bit aggressive dropping into ever lower gears as you slow down naturally or when using the brake. The torque converter will also stay in lock up much longer also. The goal is to help slow the coach down with less use of the brakes. Somewhat like using the shifter to force the transmission into a lower gear sooner.

When to use is up to you. Always? Never? Only when towing? Only on hills? There really isn't a right or a wrong answer. There are a couple of short paragraphs in the owner's manual that describes the feature also.