[77newell]

On my previous 77 coach with the 2-stroke engine there were 2 round handle valves located on the rear end of the engine on the driver's side. One valve was mounted about even in height with the top of the frame rail and the other was higher up. These are testosterones only valves in that area with heater size hoses. On the 77 coach the return water from the front core passed through the water heater coil.

If you shut off the two valves at the engine they both need to be closed or they leak out the stem.

Also on that coach the heater core was centered just below the dash and the hoses and the modulating valve were above the generator so leaks from any of those sources would leave a spot like you saw.

As always, your coach may differ.