I am new on this site and love it. I just replaced the heater core in my Winnebago. I was able to get the parts from RVmobility.com. They sent me new heater core and fan. Replacing them was quite simple actually. One thing I did and should not have was had the AC drained. In my unit that would not have been needed. It would have made it a bit trickier to work on but still possible. I would have just had to remove the front cover, remove the water lines slip the core out and replace with a new one.
Kevin