If it is just normal dust/dirt you should be able to wash it with tap water. With maybe a final wipe down with a lint free cloth and a little RO/DI to moisten the cloth/paper towel.
You might want to fill it up while it is in garage or outside (if it won't freeze) for a couple/few days to check the seals to make sure they are OK. Visually checking the seals for damage would be good also. A little paranoia now could save you a mess later.