Chemicals are not the answer. If the seals are all good, no smell will be coming into the camper.
(These are in no particular order.)
1. Is there a little water sitting in the toilet at all times? If not, smells WILL enter. If it is not holding water, the seal needs to be cleaned or replaced.
2. Sometimes, the seal between the toilet and the floor fails. Mine did once. Remove 2 nuts, lift off the toilet, replace the seal, put the toilet back, replace the nuts.
3. The AAV (Air Admittance Valve) under the cabinet by the sink can fail, letting in odors. They run less than $10 at a big box hardware store. That is always the first thing I do, replace the AAV, because it is so cheap and easy.
4. Be sure the sink's trap is full of water. If it empties for some reason, it will let odors in. The same goes for the shower's trap.
5. If there is a vent in the bathroom that is open, sometimes, when the wind is JUST RIGHT, odors going out of the plumbing vent on the RV's roof can blow back into the RV.
I have had #2 and #3 happen to me, and have had #1 happen to some friends.
Bobbo and Lin
2017 F-150 XLT 4x4 SuperCab w/Max Tow Package 3.5l EcoBoost V6
2017 Airstream Flying Cloud 23FB