I agree, a search by zipcode would be useful. I have wished it were there quite often, but did not have a reason to do that particular thing.
Now I do. I'll add it.
There are state parks, but not a lot of them. If I see a particular state park mentioned in a RV forum I add it.
The database is hand-built by myself and family members. Mostly myself. As of this morning I think there are 1,974 RV parks. I entered all but 300. I add an average of 10/day, so I expect to hit 2,000 by New Years day.
It is information I have collected over the years, plus a lot of Google searches. Google's "geocoding" gets it wrong quite often, especially in more remote regions. So I also do a lot of hand verification using other web sites, particularly the sites of the RV parks themselves. You have probably noticed that many RV parks do not make it easy to find them.
But most RV parks in my database have websites because it is much easier to get and verify information on a RV park with even a bad website than it is to get information on a RV park with NO website.
Part of the reason that I do not have very many state parks is that the web sites for governmentally run parks are quite often very difficult to use. It takes a lot more work to extract the information. Sometimes it is impossible to even get a street address or a phone number, let alone directions or a usable map. This is why I normally wait until someone mentions a particular state park to add it.
But, I expect Google to get better at geocoding (guessing position based on address). Folks in the know say that Google regularly incorporates "user contributions". Plus they mine the web. The heaviest user of ContinuousTraveler.com is Google's "bots" that suck up web pages for indexing and other purposes; mostly other purposes, since they have only indexed around 300 out of over 1900 pages. So, my hand verified GPS coordinates will find their way into Google's databases. Seems fair, since I use their maps for free.
You can add RV parks yourself, if you feel inclined to do so.
1) Register at Continuous Traveler to get a 'trial' account.
2) Follow the link that will be emailed to you to confirm that you really registered. This turns the account into a 'regular' one.
3) Go to the Publish
page (see link near the top left of any page).
4) Click the 'New' button.
5) Fill stuff in. Save frequently.
6) You can push the park's icon around on the map to position it. If you do, set the GPS Quality to one of the Verified settings before you save, otherwise the position gets automatically recomputed (I will fix this, too!).
6) Click the 'Publish' button near the top right. It will tell you if critical information is missing.
Contact me for more information or help. Yet another thing I must do is start adding FAQ pages.