I have left black tank contents over a period of several months and as long as you keep PLENTY of water in the tank so it does not dry out, there should be no problems from doing so. If leaving it that long it might be a good idea to use some septic digester in the tank (not normally useful for holding tank use) since it will remain full long enough for it to work. Even without it not likely to have a problem. For what it's worth the black tank(s) should NEVER be allowed to sit dry for any length of time whether they're in use or just in storage.
Good luck / Skip
