It is likely that your sink is vented through the roof. Probably your cable (snake) is either not going far enough down the drain to reach the blockage, or the cable is going up the vent. I would try running the line from the roof vent with a 75, 3/8 diameter cable, or if you can access the drain under the trailer, cut it and run the cable from there. If its all above the insulation barrier, I would cut it as a last resort. Good trailer insulation is like a good man, hard to find, and if youve got it you want to keep it.