The Salem 27rkss grey water tanks are not connected. Good luck in getting capacity, a friend has had one for 2 years and has been asking the same question. The manufacturer rep did not even know there were 2 tanks. We are going to fill the galley tank and empty into 5 gal buckets to get an estimate when we are together in August. I will post the results if you do not have an answer by then.
The easiest way to add an additional tank is to buy a portable "honey wagon", but you will need a place to dump. I can't imagine trying to retrofit an additional holding tank to a TT.
2014 Flagstaff 27RLWS
2004 Chevy Avalanche 2500
1998 21' Thor (doesn't everyone need 2 TT's?)