If you go to the USGS site http://historicalmaps.arcgis.com/usgs/ and enter the name of you town, and state that is nearest to the campground, look for the oldest topo map available. Then, try a couple more maps, especially, if you know the approximate date of the post card, match that date up to the nearest topo map available.
You can scroll the topo maps online, download them, print them out on your computer, or, better yet, take them to a surveryor, like your county surveryor and ask how much to print the map out in a larger scale.
If you scroll the map online, you should be able to find the campground, or, a cluster of buildings on the map, even if the location isn't identified on the map.
I have a friend who lives near a spring called BONANZA... I put in BONANZA SPRING, and his state, and the location came up on the topo map.
You may try putting in the name of your campground at the USGS site I posted, and see if it comes up... like this..
"Buster Campground, Iowa" See what happens. Now, you may have to hit the + or - sign to zoom in or out to see if you are near your town, but it just may come up. If not, you can go back to the scrolling the map method, near the town that you believe it is near.
All the best