How do I get rid of a sulfur kind of smell under the sink in my RV?

Do you know the source of the smell? Sulfur could be associate with a gas line leak...or former one ? Those small wood interiors absorb odors, and they may be a part of your RV experience! Start by cleaning the sink drain - it may have trapped something yucky in there.

first - remove EVERYTHING to be sure there' s not a rotten potato hiding somewhere .. then check for leaks-- if you 'think' it's dry... put a layer of paper towels down and check in a day or two ... if it's particle board could be wet ... and of course your holding tanks ... if you have not cleaned/flushed them in a while the scent could be from there ... the sinks and shower go to a 'grey' water holding tank --https://rvshare.com/blog/rv-grey-water-tank/

First empty the contents of your toilet holding tank completely. Next press the water pedal and get your fresh water going into the holding tank. You can speed this up by using your hose. You can monitor this by the water gages on your RV. When you get a few inches of water in the holding tank pour in 1 gallon of bleach and let it sit for about 2 hrs. Go outside and re empty the water holding tank. Go back inside and run some more water to rinse out residue that’s left in your holding tank. You have now cleaned your holding tank. Rinse again if necessary. You should have a clean holding tank now. This takes a while to do but it’s worth it in the end. Run more water into your holding tank and add a cup of bleach and flush. You should be good to go. You can also get on Utube and a viewer has a video. This process involves 1 person. Try to clean once a month to avoid having to do this as it’s labor intense. And yes, women can do this!

You may need to add more treatment (blue stuff) to your black water tank, is is gases seeping up

Most RV manufacturers vent the sink's plumbing into the base cabinet instead of piping inside the wall and through the ceiling. Your funky smell is probably grey tank fumes at their best. Try adding more deodorizer enzymes or dump the tank more frequently (I know, it complicates black tank dumping)

My kitchen sink grey tank stinks the worst. I found that washing the dishes in a plastic bin and tossing the dish water outside helps. The bin is small so it sets on the counter and I finish rinsing the dishes in the sink.

I use biodegradable soap but some parks still don't like campers disposing of their grey water this way. So be warned.