Empty the tank and dry it thoroughly. Drain the lines as much as possible. When you return use Peggy Hall's recommissioning procedure for sanitizing the system. It is posted many times in various threads here.
In case you can't find it the link below is a good recipe for sanitizing a water system. Best practice is to do this before putting the boat into storage and again when recommissioning. Leave the tank dry during storage. Also, do this process once per year when cruising.