Hi - welcome.
The only Subaru with 7 seats that I'm aware of is the Tribeca, made up until 2009. You would need the SE7 version. They only return about 23mpg, but a good number of them will already have been converted to LPG, giving an effective 40mpg in real terms.
I'm not aware of any 7-seat Foresters, sorry. Others may know more than me though.