I doubt there are any blueprints, these things are thrown together by the fine craftsmen using the vast knowledge in their heads and no two trailers are exactly alike. In the RV world, water does not flow downhill, it flows in the cheapest direction possible.
The drain valve cables are probably routed wrong and not adjusted properly. Download the installation manual for your valves.
The water spilling out is a common occurrence because vent hoses are simply run straight down, see the above posts. The time that water still flowed when stopped might have been from the trailer not being perfectly level, it only takes a few inches. The vent hoses need to rise well above water level and ideally should exit the coach at that level.
Don't be afraid to pull down the Coroplast, it should be done on every trailer so the assembly process can be completed by the purchaser. You will find unbelievable workmanship and many things that need to be corrected. Wiring, plumbing, insulation and mechanical can be improved.