If the holding tank is dumping, and it's just slow draining from the sink, then your drain lines are clogged.
If grease and food particles have been dumped down the sink, then it's most likely clogged.
Using an Enzyme eater will clear the pipes of anything blocking them as long as it is bio degradable (food) and not plastic in the pipes.
Potato peelings and washing bacon grease down the drain in an RV is a disastrous clog waiting to happen. Before washing any dishes, especially greasy pots and pans, wipe them completely out with paper towel first and work dilligently hard to keep ANY food particles from going down the drain.
With all your best efforts, some food particles will go down. It's inevitable. And it does not take much to create a clog, especially when the camper is not being used and the crud in the drains dries hard, the next batch of food particles comes along, sticks to that, dries, get's hard, and repeats and repeats until the drain clogs completely.
Enzyme eaters for drains work great for your home pipes AND for your RV pipes. This is how high pipe and drain maintenance is done on high rise buildings and cruise ships. They use enzyme eaters down the drain.
Do not use Drano or any caustic drain opener. They really do not work and can actually cause more damage than good, especially to RV plumbing seals and such. Not to mention, the caustic effects on everything it touches, and the poisonous gasses those products create.
See my post on this thread:
Click here.