If the LOW POINT DRAINS are closed, and this tube is from the FRESHWATER TANK, then suspicion would be OVERFLOW, and caused by bad check valve internal to the WATER PUMP, and would require pump rebuild or replacement to repair. (Worst case) would be a burst tank, where tank pressurized during filling, and tank cracked/ split. FW TANKS WILL NOT SURVIVE ANY PRESSURIZATION.
(TerryH.) 2000-GS Conquest Limited 6266 Class-C 99-E450SD V10