Crawlspaces are generally a problem, and hard to correct without good information. If you have foundation walls with dirt floor, and you wrap the foundation walls with plastic, the foundation walls will never breathe, moisture from the dirt floor will rise up around the floor/wall barrier edges, and you will likely end up with water running down the foundation wall barrier, and the whole cycle will never end. Worse yet, the wood above the foundation walls may rot much more rapidly.
There are many considerations, but adequate venting of the crawlspace may also alleviate the problem. The tradeoff to venting is cold, and can be insects or rodents if not done properly.
I'd also recommend putting an insulation blanket on the floor that is typically used under concrete poured basement floors. Insulating the dirt floor may help alleviate some of the moisture problems by creating a more static temperature and eliminating the effects of heating and cooling that drive condensation.
I'd definitely avoid wrapping the walls in plastic, the moisture from ground will creep up the foundation walls under that plan.
The best solution is to pour a thin 2" layer of concrete down on the floor or the crawlspace above 2" of polystyrene foam. Then you can wrap the thing in plastic without effects.