The plumbing bay thermostat is there to control the fan coils that warm that bay. I like like 50 vs 40 because it keeps the internal freshwater tank warmer making it eaiser to heat water from the internal freshwater tank in really cold conditions.
Ninety or one hundred degree hot water is a problem. You should have 115 to 120 degree F hot water, period.
The “A” plug in the Webasto control unit, the brain box mounted on the side of the burner, turns on a coolant circulating pump anytime the burner is running. Have you checked the operation of that pump.
Plus ask owners of your conversion how theirs is set up to work. Some converters run the pump via a relay that operates the pump all the time hot coolant is needed for water and/or interior heating. So in this case the Webasto could be cycled off but the pump would still be running.