I got a call from a customer who felt his motor was running a little hot in the fall and thought he should address it during the off season. We decided to pop the caps off the heat exchanger, check it out, and take it from there.
I popped off the heat exchanger caps on a warm Maine winter day, luckily before we'd had any real hard freezes. I found this diluted antifreeze.
Noticing the very diluted color I took a cup of it home and tested it with my sight refractometer. The freeze point was around 25F. I called the customer to inquire about the "winterizing". The owner had decided to undertake the winterizing himself in the fall, and claimed to have sucked 3 gallons of -50 propylene glycol through the system.
This heat exchanger could have possibly frozen and developed a leak. A repair like this would cost a fair amount of money, and certainly far more than en extra gallon or two of antifreeze.. RV antifreeze/propylene glycol is not intended to be diluted and should be used full strength. When diluted the freeze and burst points rise rapidly.
When in doubt suck in another couple of gallons or test it.