I have lived in a RV, Tent or Trailer most of my life and have also done all of the stealth camping described above (Great Advice, Some I Never Realized) however I have a few suggestions as well.
I learned early on that approaching the police is easier than dealing with them later (unless your a fugitive lOL)
I was stuck one day waiting for a western union (last paycheck) so I stopped a officer and explained that I just needed a place to park for the night until the western union office opened and I would be on my way. He was very gracious and told me about a nearby pond and vacant lot on his beat and that I was welcome to park there for the night provided I was gone by 7am (when his shift ended). After realizing this might be a good approach I began stopping in small town USA and going right to the sheriffs office or police station and never once got turned away. Some instances I even parked right there in front of the station or in their parking lot.
Later in life I became a minister and realized how many small poor churches needed repairs they could not afford. Being a contractor and RV Tech by trade I decided (My new wife and I) to go on a sort of mission. We crossed the US from PA to Florida to California and back stopping along the way and offering to help out and all we asked for in exchange was a extension cord and parking space. It was quite gratifying to be able to help so many small churches along our way from building puppet stages for youth ministry to repairing leaky faucets and sinks and we met a huge variety of great people doing it.
We even got invited many times to home cooked dinners and pot luck dinners just to socialize. In some areas I went to work at a local RV shop or temporary labor (me and the wife) for travel cash and simply traded a days work on Saturday generally mowing a lawn or fixing some small problem in trade for a weeks stay. When we had long hauls to make 99% of the small churches have the pastors name right on the sign and a simple call to his / her home asking permission to park there over night was more than adequate and even though I offered many said no need to do anything just feel free to park there for the evening.
Granted we did occasionally run into some churches that was not as hospitable, some even down right rude but normally that was not the case and after 2-3 calls we had permission. And by the way 90% of the time they never even knew I was a minister we didn't want to make them feel like that was some kind of leverage and the few that did find out was surprised because unless it came up in conversation we just never offered the information so even a regular non clergy person could do this as well.
Another place if your in a camper / trailer is RV repair shops. The large dealerships do not even notice your there because most do not open until 8-9am and if your a early riser like me your gone before the doors open and seeing a trailer or camper parked in the lot is not uncommon.
Also fairgrounds and flea markets normally have a parking area for vendors that stay over night and we have found a few remote ones that offer hookup for as little as 5.00 for a rented table with showers and restrooms. I am a pack rat and up-cycler so even if we didn't intend to use the table and donated the receipt to a person who was looking for a table... We often spent the weekends selling stuff I had accumulated along the way and socialized with buyers who also told us about great boon docking and free campsites they knew about.
We currently live in a combination RV Park / Hotel and my wife works as a housekeeper with the first 12 hrs a week going to trade for full hook up. a 400.00 per month + tax normal value. And we have worked at other campsites like this as well. Simply ask and many will say yes... Most times to ride a lawn mower for a day, maybe clean up a bathroom or take out trash.
We once had a couple (To our surprise) that wanted a week off so badly they traded a week of just emptying the trash and cleaning the bathroom for a month free hookup. They had a drop box so all we did was stop and say hello to people coming in and explain the owners was away for a day or two to use the drop box and we would help them if they had any problems with hookups or needed anything for the restrooms or showers.
They had been so delighted we stayed a few months after that working in town and doing odd jobs once a week to trade for our spot.
The moral of my story is law enforcement can be your friend if you ask them before they stop you... Most churches will extend a hand if you ask and appreciate some help if they are in need and you feel good helping and most camp ground owners have a few odd jobs to trade for a night, week or even a month if you ask.
Good luck and happy boon docking