There is a way to fix this issue. Use full hook-ups and leave the galley tank open. But your sewer line needs to have a bend in the line to form a elbow similar to a sink drain.
I do this all of the time with our tank that drains the shower.
You need to use a sewer hose support system first. These are plastic expanding supports that are inexpensive to purchase. You than run the sewer line for some distance away from the trailer and allow the line to dip between two of the supporting members. This is were the elbow is formed allowing water to be trap in this dip, similar to a bathroom sink drain. But you need to remember when emptying the black tank to remove the dip in the sewer line. After emptying the black I will empty my other gray tank to clean the line and put the elbow back in the sewer line.
2016 34RL CC; 2008 Ram Mega Cab 2500HD, 6.7L, 68RFE 6 speed, 4X4, Smarty S67, TDR 135K+miles