Don't have a Newmar (yet). But as said disconnecting the battery cable after fully charged should work. Or keep it plugged in. The little solar charger isn't made to charge a battery when needed. Just to help keep it topped off when charged fully.
As the others said your generator should be able to be setup to come on to charge your batteries as your require. I finally set mine up to do this one month ago, as I have a problem with the onboard surge protector shutting down shore power when it shouldn't ( needs to be fixed). Just remember every 30 days you have to reset the auto run feature for this.