I didn't know the answer whether it's legal to camp outside camping until I read your thread and decided to look for the answer in the Internet :)
According to turistic forums in Polish that I've found, it's illegal to camp outside camping sites in Poland, you can get a ticket fine for that. However people seem to do it despite it. As for camping on someone's property you should ask for permission the owner. People say that usually they had no problems with that, of course you need to look decent not like a tramp.
Hotels are too expensive, that would be a great amount of money for a 12-13 days that i'll spend in Poland.
What about hostels? In many you can pay just for a bed if you don't mind sleeping with strangers (actually it won't be the full season, barring the national holidays 1-3 of May, so probably there will be lots of empty rooms).
I'm a citizen of Ukraine.... but i'm very nice, clever, good educated guy with a good manners. I'm never gonna do any "bad" things on a foreign land. I really respect people and nature, wherever i go. But still i'm a little concerned about my nationallity....
If you look like a backpacker not a person with suspicious intentions then I don't think you will encounter any troubles.