The Lonely Biome by the same authors might help you do just that, but I am not 100% certain if RTG works in a Lonely Biome World.
Another way is using Biome Tweaker:
1. You need to remove all biomes except mesa / set their gen weight to 0.
2. You need to add mesa to all (four) climate groups.
However I still don't know if this leaves you with some biomes that can not technically be removed. If so, maybe you could try setting the Mesa gen weight to something ridiculously high.
It seems to work with Lonely Biome. I still might try Biome Tweaker as you said just to see if there isn't any functional differences in how the final product looks.