I know there are more options out there, perhaps better ones even, but this is about 1/2 price, and if you are charging while towing, or have a 12v receptacle in the camper, this may be helpful to knowing the battery level....

http://1saleaday.com/flash/rp-203vm/

FREE SHIPPING!

Model: RP-203VM

Features Include:

    Plugs directly into your cigarette lighter socket to indicate battery condition accurate to 0.1 Volt
    Converts one socket into two sockets
    Display dimensions: 2.5 inches wide by 1 inch tall
    Adjustable neck for easy access