Drainpipe coated with lime

I just bought a trailer with a problem that has me stumped. The bathtub faucet must have been leaking for years and the drain has lime buildup on it at least 1/8" to 1/4" thick, it looks like a stalagtite all around the drainhole and I’m sure the rest of it is just as bad, if not worse. Because of this the water drains very, very slowly - I don’t know how the previous owners used it without flooding the home. I tried Draino but that was useless and I’m not sure what else I should try. None of the sinks have this problem so it’s just localized to the tub drain.