Ok, well, yes, check the heater lines and see if they are connected. If they are, then verify your valve is working, hot on both sides when heater is on. If you have owned the coach since it was new, then the heater core bypass is probably out. Heater cores for moho's are crazy expensive, and the labor to install them is even worse because frequently the box they are contained in, have disintegrated, like to the tune of $1200. I have used foam and aluminum tape to make repairs to these in the past when I couldn't find the parts to replace them. Very labor intensive...... But that said, start at the valve, make sure it is getting activated. Often times they are vaccum operated and a leak will make them useless when an unrelated dash repair is preformed. Lastly, your outside vent door could be stuck open. Lazy way to test it is to tape it over with some ductape. Good luck, nothing worse cold feet.....
May your black water hose never break!