Kev, I took the liberty of moving your question to it's own thread. You should get some replies soon. From looking at your E30 floor plan, I would think your water pump is under the kitchen cabinet or the bath vanity.
You mentioned 2 blue drains. Like you thought, one is probably the fresh water drain. he other may be a line to use the water pump to empty the tank and system. Sadly, this is something your dealer should have covered with you when you picked up the trailer. Please feel free to call Heartland and have your VIN. handy. They should be able to help as well.