Hi Tomsboat,
Go through all of the setup parameters on both units...one of the parameters is to 'reverse' the fan speeds when in heat mode. If I recall correctly, it is P13. Make sure it is set at the default. Check the other setup parameters as well, they should be the same for both units.
On my Mainship 390, I have two (new) Dometic Marine Air units. The way I have them set up for heat mode (factory defaults), leaves the fan running slower when actual temp is more than 5 degrees lower than your 'set' temp. As the boat warms up inside, the fan speed >>increases<< as the actual temp approaches the 'set' temp. When the difference between actual and set is less than two degrees, the fan should speed up to maximum.
I know this seems counter-intuitive, but there's a good reason for it, which I believe has something to do with the Second Law of Thermodynamics ;-).
So in heat mode (only), when P13 is set for 'reverse' (as it should be), fan speed is inversely proportional to the difference between the temperature you want and the temperature you are at. My guess is that this is what your are seeing.
Hope this makes sense...