Mine is the same way, it doesn't turn off all that well.  On a recent
hot trip I found when I got into camp and went under the back seat to
get something, everything under there was hot hot hot.  I checked the
valve and it was in the off position, but apparently enough hot water
was still going through there to heat that core up plenty hot.  Maybe
an alternative valve could be put into the system instead of the
stock one?  Enlarge the hole in the heater cover enough to be able to
operate it?  Anybody done this?
Edward