By chance I noticed an entry in the SGP focuser dropdown focuser box called "Robofocus Server". I read up on it and it turns out that this is an alternative way to drive the Robofocus focuser. Instead of creating and Ascom interface, this driver starts the Robofocus control program and uses that. One big advantage is that this driver supports reading the temperature sensor of the Robofocuser unit.
As I found out previously, the Robofocuser unit does not work with the Serial-USB adapters with the captive port. So, I used one of the cheap ones (which does not retain its port number) and it works beautifully: connects, controls and reads the temperature.
One weird thing is that the temperature is 100*Celsius...
For now, I will just use the Robofocus Server for focusing, but keep using the TEMPerHum device for temperature reading - then I won't have to recalibrate my temperetaure/focuser coefficient. If it works, then I should try to find a Serial-USB adapter with a captive port that works with the unit and ultimately might be able to get rid of one device.
Or is it better to have another device, but one program less running on my laptop?