Personally, I have never seen a bypass system with only one valve. There has to be two lines to the WH, and there should be valves to shut off both lines. The valve you found sounds like the one that's usually between the hot and cold lines, which should be opened for winterizing. Could be something new and different, but my feeling is that since the one valve you turned to "winterize" did not prevent the WH from getting antifreeze, the other valves must be somewhere else and out of sight behind something. Try tracing the input line back from the heater and see if you can find another valve that way. Failing that, could you check with the previous owner to see if he knows where they are. Lastly, contact Jayco and ask them to email you a schematic for the water system that shows the bypass valves. They are usually pretty good at sending that kind of info.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.