Inverter help

I run a "poor man's transfer switch" and have for years and with several different RV's. You need to isolate the factory charger with a breaker so you don't run a charging loop and drain the batteries. Also the OP does not say how the inverter is wired to the batteries and what size cables he is running. You don't need a transfer switch to make this work, but some wire sizing and voltage readings would be more than helpful.