Hello jan & dan, i work in pest control and have dealt with a lot of infestations within caravans, the majority of the time i have found the mice entered through gaps around the waste/gas pipes under the van and also through the air vents from the fridge etc. mice only need a gap about the thickness of a pencil to gain access and once they get a hold on the slightest crack/gap they will make there own entrance(2-4mm mesh on inside of vents will deter further ingress just check with manufacturer for airflow requirements and if sealing large holes fill with wirewool before sealing), i would recommend placing several nipper traps (twix or peanut butter is best bait) throughout the van to ensure they are not harbouring within the fixtures or wall/floor fabric, using poison is a longterm treatment but it doesn't hurt to have some down also. Hope this helps, Ratman