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.
Alternative is that the water valve that brings engine (radiator) heat to the HVAC system is stuck closed. Neither of these is typical of a 2018, though. Could also be a simple control problem, e.g. factory forgot to hook something to the dash control. Just have to dig in to find it.
I don't know Fleetwood, but I'll hazard a guess. On DPs I'm familiar with there is an electrically powered vacuum pump which provides vacuum to operate the air control doors. You can find the pump and its reservoir on the firewall. Checked to ensure you have vacuum.
When I purchased my used Class A from Texas the previous owner had installed a manual valve in the heater supply line that was turned off. I guess they never used the heater. Once I found the valve and opened it my heater worked fine.