Another thing to check is that there is a coolant shut-off valve on the inlet heater hose to the heater core to keep coolant from entering the heater core when the air conditioner is on. (Just a little bit of hot coolant can negate the A/C.) On some rigs (mine included) that valve let enough coolant past when it was shut off that some owners (me included) added an additional mechanical valve in series with it to completely shut down the coolant flow during the summer months.
For the most part, your system is probably similar to most automotive heater/AC systems.
Good Luck,
~Rick