In Finland they have solenoid kit,
http://www.cnghouse.fi/tuote/55015_volvo-pullohana/ if its really the case. This is for the biggest tank, you might need other 2 for smaller ones, or no.
Now just brainstorming here, but idea that you have 3 tanks, 1 big, and 2 smalls. Not exactly sure how they works, but if its the case that one opens only after other one is completely empty, then my guess is that the problem with opening valves of 2 tanks :?