You need to place the monitor's shunt so all the negs go to it first, then the single fat wire from the shunt to the neg batt post. That includes the neg now going from the house batt to the frame. If comes off the neg batt post and goes now to the outer end of the shunt.
Are the power centres getting confused here which is for what? A friend has a Sunseeker where the converter is on the back pass through front wall, connected to the power centre inside on back of that wall by the bed. ISTR some other power stuff was down there by the steps in an outside compartment. Your have alternator charging to the house batts through that one, not to be confused with converter charging via the other one.
