Welcome to the site.
If you're plugged in the converter will charge the engine battery (make sure the use/store switch is "on").
In fact, whenever you're plugged in use/store should be in "use".
I put a knife switch on my engine battery and only disconnect it when I put the coach in storage and not plugged in.
If you haven't already you may want to download and read Ed's excellent crowd sourced Axis/Vegas owner's manual found here:
https://www.thorforums.com/forums/f1...nual-4961.html