I'm afraid any filter you try to install over fridge vents or water heater vents will just create heat build up and a lack of ventilation. Better bet might be to have a small vacuum cleaner with hose that you can use to clean them out if you notice a problem developing. In 5 days, unless you encounter dust storms, I don't think the dust will build to a level to cause malfunction of either device, plus you've got the vacuum to reduce any build up.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI