The drains need to hang low, or you'd lack the neceesary angle. The tanks are between the rails and the drop from the bottom of the rail/bottom of the tanks to those pipes. And then you need to concider whether the trailer is level affecting the angle of the drain pipes. I've noticed after my black tank drains, after I drain the grey, I reopen the black and more sewerage comes out. I've always questioned this and assume it has to do with the weightredistribution caused by dumping the tanks. 80 gallons of grey weighs nearly 600 pounds. That black tank 300 pounds also depending on fiber content
Should there be a problem I am sure Crossroads would correct it though I'm sure a lot of engineering went into those drain pipes.