When I replaced the HW heater on mine I had to pretty much drain the system and, if I rember, it took a tad ove 1 gallon to get it back up and running. Also, if memory serves, the capacity is 6 qts.
Is you'rs low on coolant??
The manual called for 50/50 mix as I recall. Anyway, that's what I used and I guess I should be looking for some P.G. to replace the E.G. Works the same but Joels comment about a leak in the heat exchanger and E.G getting into the Potable water system is correct....not a good thing.