Try Googling 'Beach Camping in Thailand' or 'Beach Camp Sites in Thailand'.
I did find this beach camping article which maybe be of interest: https://iamkohchang.com/activities/camping-wai-chaek-beach.html
I live in Hua Hin and I know that there are several beach camp sites in Cha Am and Pranburi which are located in the west coast of the Gulf of Thailand. They be probably be pretty much desserted until the weekend Bangkok city crowd arrives.
Good Luck