[Huck]

One thing to remember is that the burner in the furnace is not always on.
The thermostat tells the burner to turn off (and it does)a while before the fan and/or blower turn off, giving the imprecision that the furnace is running too much.
I hope this explanation makes scene.

I don't know of any way to adjust the temp. differential on the thermostat in an RV. Some home style thermostats have an adjustable setting for that.