Hi matey,
Im not entirely sure if you can re gas them. If you havent found all the lovely helpful tips about fridge servicing in the WIKI guide ^^^^^^^ up there look, the take you fridge out of your van (if you can) and turn it upside down overnight, then back the right way again overnight, the service all components of the fridge. ie, cleaning the gas jet and exhaust flue, checking all earths, checking the heating elements of 240v and 12v work independently.
Best of luck, and check out the Wiki guide. Its really good
