How can I fix the sewer smell in my bathroom?
The two most common sources I've seen are dried out traps and a failing wax seal under the toilet.  A failing wax seal may also result in water damage appearing from beneath the toilet.  The dried out traps are simple to fix, just run a little water into each drain every few months.

For a, more rare, problem at a PVC joint, you'll need to find the offending joint first.  Look for signs of water damage, or you may get lucky putting some soapy water over each joint and flushing a toilet.

Edit: here's a good description of how to replace the wax seal under the toilet: How do I fix a leaky toilet that only leaks from underneath?