Depending on the level of experience you have, the correct way would be to replace the countertop altogether. The particle board is already compromised. This will allow a fresh, smooth surface to re-mount the sink. Countertops are pretty cheap at Lowes and there's not much of it in an RV. Also a thought to consider, does the sink have to be an under mount? While it might require swapping out the drain line, you could mount it conventionally on top and eliminate having to deal with it again. Rv's bounce around a lot and and undermounted sink just seems like it's asking to pull apart again.