If I understand correctly, your leak is at the base of the sink drain basket where it mates to the drain pipe ("collar nut"). The standard seals used on homes and RVs are a hard poly washer (translucent white). They do not seal well when the drain pipes are not perfectly aligned. I have used polypropylene seals in this instance:
https://www.homedepot.com/p/Everbilt...195C/205153791
HD also sells the conical washer seals made of the same material under the Everbilt name.
They are flexible and seal well.