They are located in the HVAC ductwork, not sure where on a Class A. On a GM truck, they are under the dash. There are little doors inside, that open and close, based on what the dash controls demand. If it needs heat, it opens letting air flow in from heater core, etc. Looking at your unit, 2018... it would be odd that one malfunction so quick, not unheard of though. Certainly you are still under warranty, but I understand hoping for an easy, diy fix. Hopefully someone with a similar coach as you has some input. Good luck.