Not sure about the Palazzio but our Thor Hurricane has an Evans Tempcon HVAC unit in it. It has no temperature blender door as mentioned above (similar to an automotive type) but instead uses a hot water valve to regulate the temperature. IMHO, it is a terrible system but the valve is near the HVAC plenum. On my MoHo, it's up front under the hood and looks like this...
The valve is controlled electrically (not vacuum) by your temp selector switch on the dash. The hose should be warm on both sides of the valve. The valve can be removed and bypassed to test your system. These valves also leak and many people have installed a common heat valve (from the auto parts store) in parallel to the Evans Valve, that can be shut off in the summer to get maximum A/C. When the Evans Valve leaks, the A/C doesn't work well.
What is controlled by vacuum on this system is the solenoids that control where the heat (or A/C) is going, ie. defrost vs. floor, vs. dash. If there is a vacuum leak, the default is defrost. It takes vacuum to select floor or dash. The vacuum is supplied (on my coach) to the Evans HVAC in the right front wheel well from the engine and goes through the firewall area near the right headlight. My engine (chassis) battery is also in this area and acid ate through the vacuum line. Good Luck
No dash heat in my 2019 Palazzo 36.3. Blowing air but zero heat. Got pretty cold going down the highway yesterday. Any ideas?
Cummins 6.7 on a Freightliner chassis look for one of these valves on the top of the engine. It controls the hot water flow into the heater core. Designed to turn off the hot water in the summer thereby further assisting with cooling. Mine was turned off at delivery and I had no dash heat until I found out about the valve. Mine shown is in the open position allowing heat.
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2020 Entegra Accolade 37TS
2019 Jeep Grand Cherokee Trailhawk (Toad)
FMCA - F432054