I agree with everyone here, and wanted to throw in a suggestion.
RTP can look great in the right hands. Check out some of Luiishu's mapping in his Castle Oblivion games. He knows that RTP inside and out, and his games look fantastic. Similarly, Liberty is excellent using the RTP, and often she will make edits of the RTP to use in her games. The RTP is rather comprehensive, and all of the art is consistent. It has most everything you need, and that which it doesn't, can usually be solved through simple edits.
Don't get discouraged just because you looked at the screenshot thread and saw awesome custom artwork by people like Pizza, because they're chiefly artists. Not every developer can do that, but you can use the RTP to its fullest extent and make a great, functional game.