The sink p-traps on a travel trailer can get pretty funky smelling. You might put a teaspoon of bleach in 4 - 6 oz of water in a measuring cup and slowly pour it down the drain and let it sit for a while. When conserving water the P-trap doesn't get flushed out as well as it does on a sink in the house. Then again P traps and houses can start to stink a bit from time to time.
They do make deodorizing tablets or pellets the size of marbles you can drop down the drain, let them dissolve and sit for a while. There are also deodorizing sticks that you can drop in the drain that don't flush through and dissolve slowly over days this will keep the drain smelling fresh for longer periods of time.