Kenn, you need to tell us more about your vehicle - make, model, year, etc... A good thing to do is add that information to your signature line. Lots of good help on this forum that will get your problem sorted out once we have more to work with.
__________________ The things you own control your life.
2021 Winnebago Vista 35U towing a 2022 Jeep Cherokee Trailhawk 2010 Cadillac SRX 2003 Itasca Sunova 30B
could be a stuck open thermostat or the connection to the heater valve are off. Dont know about rv's but our buses have a separate valve to open close for heat control. some have a valve inside others have a valve out by the engine. Have you had this rig long and used heater before?
I forgot to say the crack will be where the black rubber hose connects to the a vacuum male port. So follow the 1/8" or 3/16" hose to the end... cut off the 3/8" end and reattach it.