My RV's AGM batteries can be topped back up to around 90%-95% every 3rd day in about 3 quiet V10 engine idling hours.
Barring those rules against engine idling ... at around 0.70 gallons per idling hour, you're about right on the $10 recharge cost - which I consider a bargain every 3rd day of camping in some pristine NF fully shaded campsite right next to the tent campers - with them wondering why (if they weren't out hiking mid-day) they thought they were hearing my wispering V10 runing every 3rd day for awhile. :C
The ultimate flexibility solution for RV battery recharging is to skip the solar and instead opt for the methanol fuel cell I gave the link to.