Should new septic system be filled with water?
Yes the system should be filled with water and the installer should have done that. There is a good chance the tanks can float out of the hole if it rains heavy when they are first put in if you do not put water in them. Your second question was valid The average person uses 70 gallons of water per day so if its just you and you only use the house twice a month it will take a very long time for the system to fill up. As for as gases it will be very little do to not much waste entering the system. Also the systems theses days do bot have those issues we precast tanks and have never seen one do this do to gases most break do to a car driving over them or being dropped on installation.