Although well intentioned, the poison idea is not one of the best, in my opinion, especially for the caravan. (I'm pretty sure your caravan is our travel trailer here in the U.S., no?). The problem with poison is that it kills the rodent by causing them to bleed to death internally. It won't work on the baby mice since they have no way of ingesting it. Additionally, when the mouse does die, he's likely to crawl into a nice dark, warm area and you'll be able to find his rotting carcass only by following your nose...
Check closely all the inlets into the trailer, such as the areas around water or drain pipes. Where they come through the floor or walls are always suspect mouse entrances... same goes for furnace fuel inlets and exhaust outlets. Be sure these are sealed. You probably only have a few and I'd try some traps baited with peanut butter. Check them daily and reset... Best of luck!