Thanks for the suggestion of the MH Hall of Fame Library--I just sent them a message.
I will look for an overflow under the rig near the freshwater tank. I'm just a careful person so I'm worried about exploding something if I overfill it.
I wish I had the type of fill that you just stick a hose into until it overflows, but no such luck. It does have a way to use the water pump to suck antifreeze out of a jug when you go to winterize it, so perhaps I could use that and some 5 gallon buckets to measure.