I can not give a lot of information about hitchhiking but have spend a lot of time wildcamping in Europe. Sure most land is privatly owned, and most countries have regulations against it, but in most cases you will be fine and undisturbed if you take some things into account:
- Try to hide your tent, only set it up right before you want to go sleep.
- Find a location you don't expect people to show up, little pass by traffic, remote as possible. The closer to a city/village the harder this gets.
- Don't use more light (when dark) than required, don't make a fire.
I've done this 200+ nights on roadtrips, travelling with a car makes you more mobile to reach rural places, but limits you to roads. When hitchhiking you can be asked to be dropped off and get away from the road on foot. Try to have a good map so you know in what direction to find rural terrain. Start looking for a place to spend the night a few hours before sunset, so you know you have enough time and do not need to find something in the dark. Mountain terrain is often a bit harder because there isn't a lot of horizontal ground, unless you want to visit the Pyrenees I would suggest you try to hitchhike across them in a single day.
Parking lots of nature parks or walking trails are usually an easy target, but may also attract other people, pitch up your tent where it is safe (you don't want to have a camper park on top of you in the middle of the night).
Try to not sleep near a big road, there will be traffic the entire night and you won't get a lot of sleep.
In the south there are many semi-dry rivers which are attractive and fun to sleep at. I've done so a few times but in retrospect one should really take into account that there may be flash floods, and that is definitely not something you want to happen.
If you want to spend a lot of time in cities, still finding a suitable camping spot may be hard, couch-surfing may be a good alternative for the nights you want to spend in a city.
Some other information about wild camping can be found in these questions and answers: