If it was a 1 1/2 inch line, and he has has repaired it, it was the GRAY line. The absolute best test, is to fill the gray tank until the shower has water standing in it, (assuming the shower or tub is the lowest point). Now walk away for a day, and if the shower still has water in it 24 hours later, no leaks. Good luck.
By the way, the black tank has 3 inch drain pipe
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT