Sewer/black tank smell inside trailer

Thanks for all the advice. I forgot to ad there is no smell in the bathroom of the trailer so that might eliminate the toilet. There is a vanity sink and the kitchen sink that can vent bad smells directly into the rig. The bathroom only has a shower and a toilet and the door is kept closed. I did not know that water in the p traps acted as a vapor barrier. I will make sure to fill the p traps before it gets parked. And just for good measure I will buy a toilet wand to clean the black tank. Plus the air admittance valves might need replacing as suggested. Thanks again.