top one is air intake on earlier Mk2 to allow the tank to drain quicker, bottom left is the drain plug so I'm going for the bottom one.
They say to release a wheel nut or bolt heat it up.
so it might also work with the drain valve causing the plastic to sweat, however its plastic/nylon so for obviouse reasons use a hair dryer try to ease the remains of the plug around with a flat blade screw driver or gradually break it away. To get the right tap size to cut new thread you need to buy the same type (there are two types).
Alternatively if you know the age of the caravan it will give Truma a chance to tell you the correct part number. They took over carver products. www. truma.com they may give idea how to remove it.
If successful then coat the new valve thread and rubber seal with mineral oil. I do same with the Crystal water filter and made up a block of wood to fit the filter tightening fin cutting a slot in the timber to fit over the fin and its whole length so its easier to tighten then losen without snapping the fin.
with the drain plug slot made a blade to fit snug in the slot then arraldited it into a peice of wood. Two handy home made tools for the job.
bob
April 2013