I have been to Thailand a few times but have seen very few people "camping" with tents and the like. In some beach areas, particularly in National Parks the local authorities provide a shelter in which people can sleep over night and hang out by day. I am sure you will not have any problems camping in your truck in quiet places. I have always found the police and locals very accommodating. A polite question to the local shop keeper mmay help if you are wanting to stay near town. Almost every village has a common area that is used for markets and these are probably available on non-market days.
On place we know there is camping is just south of the Phuket airport directly on the beach, either side of the road. In the country side any laneway off the main road is a possibility.