There is a device made for this exact use.  It is called a converter, and there is one in most
any motor home.  Any decent RV parts house will have them.  Try Camping World.
www.campingworld.com.  They have them at various prices starting around $180.  Don't confuse
them with invertors.

Bill in the Southwest

----- Original Message -----
From: "Robert Prouse" <rob@PROUSE.ORG>
>
> In my last van, I took an automatic 50/10/2 amp battery charger and
> wired it into my electric system so that when I was on 120v, I could
> plug it in and it could run the 12v system like the interior lights and
> stereo without running down the battery.  The charger had aligator
> connectors which I cut off to wire it into the system.  When I was using
> the stereo, I would switch it to 10amp and it would keep me charged.
> The 50amp setting was good for a boost when the battery was low and the
> trickle charge helped to top me up when I needed it.  It seemed to work
> fine and I have been thinking of doing it again, but I want to know if
> it will do any damage to the battery or the electrical system wired in
> like this.