After spending some additional time with the POI Mega File, although it
looks very good for public campgrounds it seems to be a little short when it
comes to commercial RV parks.  It shows a lot of the Members Only campground
systems like Thousand Trails but looks a little short on covering commercial
RV parks in general.  Maybe they are hidden under other headings, I'll keep
looking.
BUT, in the process of messing around I found this Website:
<http://www.uscampgrounds.info/>
And it looks terrific,  says it's got all public campgrounds and provides
downloads of files for your GPS or mapping software.