My drain tube (Dometic) extends several inches from the chassis in front of the rear wheel. (below the refrig) It terminates in a small perforated plug. The plug twists out and reinserts easily. That is where I figure any stoppage would occur. If further up I would start by applying suction to the tube and if that didn't work perhaps some low pressure. Good Luck
Jan and Rocky
Volunteers for USFWS. 29 refuges with 9300 hrs ea since 2006. 2004 Allegro 30DA, Workhorse 8.1, Banks, 2012 Jeep Liberty Jet, Blue Ox Aladdin, Brake Buddy Advantage Select, 300 watts solar, 5 Optima group 31 AGM's, 2000watt Ames PSW inverter.