2012 Coachmen Mirada 34BH Heating Fresh/Holding/Grey tanks

Not sure on your model, but on our 2014 Coachmen Pursuit the black tank sits right under the toilet and the gray tank runs crossways behind the black tank. The wet bay has the furnace line blowing in to warm the compartment. They took the 4" line and put a tee into it. Then there is a 2" line ducted down into the compartment.

You might have to remove or move the panel in that compartment to see were the tanks are. At the least to follow the piping.