Because of all the problems I have heard of with plastic and fiber tanks underground, I would try as hard as possible to get a concrete tank. The concrete and a crane to lift over your house could be less than a fiberglass tank.
Take a look at your local farming supplier, they usually have large tanks that are used for animal waste and might be allowed for sewage systems.
You could also have a fiberglass tank manufacturer custom build one for you. There are companies across the US that do custom tanks all day long and ship them to the customer via truck for all sorts of purposes.