Yup--12 volts is 12 volts but be aware. The average auto bat charger is 6-10 amps while your Xantrex inverter max charge is 100+ amps. Depending on yr re-charging needs, you will have to run yr Honda a long time to recover yr bats. Yr Onan genset was built to be used--1-2 hrs in the am and 1-2 hrs before bed isnt a lot to ask. Besides, you should be running yr Onan one hour per month anyway--so just pretend "its yr time of the month." Your call.
2003 Alpine 40' MDTS
New Braunfels, Texas