It takes time but I plan the routes in advance and know how far we will want to travel on any given day and then in that area I use Google Maps to do a search around the area for camping locations. I get mapped locations for private RV parks and public campgrounds.
The states all operate websites for their state parks and the federal government also has websites for BLM (blm.gov) the US forests, and national parks. In a state like Utah for example there are state and federal parks, BLM (Bureau of Land Management) lands with camping areas, and national forests.
For private RV parks what also works is to use www.yellowpages.com to do a search for campgrounds for a specified town and it will provide a listing of contact information and display them on a map. This also works for finding restaurants and service stations that pump diesel or when looking for a grocery store or other type of business in an area.
I use yelp.com a lot to look for restaurants in an area and 80% of the time it is useful for finding a good place to eat.
I have never in my life had to stay in a parking lot anywhere at night in four decades of travel. If you like to stay in such places then go for it but it is in no way necessary unless you are too lazy to look for a decent place to stay.
