Here's a respected site for all things RVing in the U.S.
http://www.ideamerge.com/motorhomes/usa/resources_2.html
Additionally, I see a lot of 'campers' using the parking lot of Walmart stores. The above website has a link to using Walmart as well as a free app to download for that puprose.
The cost for overnight trailer park sites with electricity, water and sewer hookups varies greatly... from as little as $15 nightly to over $75. Our National Parks have free sites with limited hookups and there's a link to that service as well since it usually requires reservations.
Good luck!