I have 11 tanks,(2 x 180L, 1 x 125L and a John Allen set up with 8 x 55-60L's) in my garage. It is built on the side of my house and has a pitched roof which is boarded so I can store all the camping gear, fishing tackle and alot of junk in the rafters. I have an insulated roller shutter door and that keeps the garage at about 22C all winter. I will in the future get around to insulating the roof joists up in the rafters but I have to empty all the stuff out and that will not be easy.
I found the insulated roller door a great help but it did cost £1600 for a single width garage.