I really hate Dometic for killing Attwood so they can sell more dometic **** when you can't get attwood parts.
Here is manual for your 8900 page 11 top
It looks like your thermostat just connects 2 wires from heater to turn heater on. (it looks like both wires at thermostat are blue).
so if this is true, the stage 2 heat is triggered by the control board depending on how long the burner runs.
your thermostat may also control your AC, there is 2 wires your connecting together with your thermostat. easy to wire with a standard household thermostat.
the trick is that the AC might have 2 fan speeds. I have had to add a small toggle switch by drilling a small hole in thermostat to mount a small toggle switch that is wired to control the fan speed.
but in later years Attwood (and I am sure others) started using a 4 wire buss between the AC and thermostat.
That and RV heatpumps is where you start to have problems wiring home thermostats.
Here is another manualsee page 14 and 15 of this manual. it has complete pin out for your dual stage thermostat
you should be able to duplicate this with a home thermostat, but you will need to add a toggle "fan switch" so you can switch AC fan between low/high.
again, I do not see a 2 stage controller so this must be done by control board in furnace. I recall having one in a Titanium camper and it worked well.