Atwood Hydroflame RV Furnace Repair and Basic Operations

I had an owner of a Terry contact me about a furnace that sounded awful when running. Now this is 20 years old and although I recommend a new one, he chose to fix what he had a 8520III Atwood Furnace. So join me as I fall into another rabbit hole and troubleshoot a vintage furnace.....spoiler the new ones operate the same way. Thanks!