Land of enchantment

I have a 1990 Itasca Spirt, I'm about to connect it to my Sportsman 4k watt generator. I noticed on the aux battery compartment it says to disconnect before attaching to external charging. Do I have to just disconnect the aux battery only or also the engine battery? Also will this generator work or do I need an inverter generator? (It has an 120 RV outlet on it.) I'm in the mountains with no electricity yet. Thanks in advance.