Forget about trying to run the cab AC without the engine on. All you will be doing is pushing hot air into your motorhome. And yes, you will be putting extra demand on your batteries although your inverter will be recharging them when shore power is plugged in. Your truck's AC is only designed to cool off the cab (driver and passenger) when underway.
You already said you have 2 AC units and short of adding another one (which is an overkill), it sounds like you have more than enough AC for your unit.
If you are plugged in, run your AC units, close your curtains, keep your windows closed and you should be comfortable enough. If you want a 62 degree room, reserve a suite at the local highline hotel.
