Originally Posted by TeddEBear
The sliding doors under the kitchen sink and to access the heater beside the door.
Ah, OK. Well, have a look at this thread. There are lots more, but this one gives the basics.
If in general, your trailer is really dirty, this could be part of the problem, too. There are a couple of people in that thread that talk about how they removed the doors to work on them, but failing that, maybe just getting a box of Q-tips and a pan of warm water to clean out the bottom track could be a good start.
Then when it's all dried out, give that bottom track a healthy shot with a can of silicon lubricant.