100% tinfoil hat. Casinos cant change the RTP of games. Its impossible. This is fact.
Game providers do make their games in different forms, well some do. Example Book of dead is available in 96.x% RTP, 95.1% RTP, and 91% RTP. The casino can choose which one they get but the RTP is shown in the game itsself.
Even small casinos make 20-50k a month form just 200-500 regular players, so there is simply no need for game providers to be rigging it for the casinos. As for game providers, check the stock market for the big ones, they are the ones raking it in.