We’ve always lived by the rule that it’s always better to ask for forgiveness than permission. That usually works fairly well for most situations, but when it comes to looking to park your RV for free overnight it might be better to ask for permission first. 
Then you can sleep soundly with worrying about someone bothering you in the middle of the night asking you to leave or getting issued a trespassing ticket.
Here are a few free RV parking possibilities:
Highway Picnic and Rest Areas – that’s what they’re there for!
Truck Stops – you usually don’t have to ask permission but be considerate of the big rigs.
Walmart – not all stores allow overnight parking. It will depend on city ordinances. Check with the store for the policy.
Other Big Box Stores – like Kmart, Best Buy and Sams Clubs will often allow free overnight parking.
Fraternal Organizations - most VFW, Elks, etc. may allow you to park overnight if you’re a member.
Fairgrounds – if there’s not an event happening, you may be allowed to camp out in their parking lot.

Cracker Barrel - usually tolerant of overnight parking.
Cemeteries – If you’re not creeped out by sleeping with the dead , go for it! At least they’re quiet!
Churches – Might call ahead for permission but most will allow you to spend a night there.
Parks – Many city, county, state and community parks allow overnight parking but it’s best to check the local laws first.
Movie Theaters – you can always claim that you were too tired after the late show to hit the road.
Truck Terminals – may allow you to park in the area for large trucks with permission.
Hotel Parking Lots – try to find one that has a restaurant and bar. You can claim you had one too many while singing Karaoke and couldn’t drive!
Vehicle Repair Shops – Needed a repair right away in the morning?
Grain Elevators – there’s usually no one there after dark anyway. Just park out of the way and be prepared to leave early if asked to do so.
Nightclubs - Another instance where you can always pretend that you had one too many and needed to sleep it off!
RV Dealer – you’ll fit right in, especially if you’re driving one their brands!
That’s just a few of the obvious and maybe not so obvious places to park your RV overnight. Keep in mind that if you are not asking permission it’s best to not draw attention to yourself. Don’t put out the awning, extend the slides or fire up the grill! Most importantly, be aware of your surroundings and if you have doubts about the area, go somewhere else.
Here is a link to an article by frugal-rv-travel that offers some other tips and information on free overnight parking.