Websites for finding RV parks and campgrounds
Campendium - aggregates a large number of campgrounds and places to park overnight
Campspot - a service that runs a booking platform for the RV parks listed on their site
Camplife - a service that runs a booking platform for the RV parks listed on their site
Hipcamp - find niche camping spots on private land
RV Parky - aggregates RV parks with places to park overnight
Facebook Groups is another great resource for finding an campground. Search for groups in the area you're looking to stay in, or join a group geared around your type of RV and others are likely to have recommendations based on your type of RV.
Membership programs
Harvest Hosts - a program that allows you to stay at wineries, farms, and other unique locations for free, usually for a single night
Thousand Trails - NOT RECOMMENDED
Thousand Trails offers a membership program where you pay once and get access to a limited number of RV spots for an entire year. However, we strongly do not recommend this program because of the many restrictions and limitations that come with it.
For example, you can only stay at a park for a certain number of days before you have to leave for a certain number of days. They also restrict the number of campsites that are available to the program, so even if a campground has availability, the spots may not be available to Thousand Trails members.
Before signing up for a Thousand Trails membership, we recommend doing your own research to decide if it's a good fit for your needs. A great place to start is the 30k+ member Facebook group, Thousand Trails - The Really Good And The Really Bad
Other useful services
RV Spot Drop - find canceled or unsold RV spots