Nascarcruzin, you are correct, your rig originally used either the RV-CP or Intellitec BCC box that has the circuitry (called a Bi-Directional Relay Delay {BIRD} function) that controls the battery isolation/charging from alternate sources. (i.e. alternator or converter)
Your problem may be either the above referenced circuitry or the solenoid the circuits control.
QUESTION - Does your aux start function work? Pressing, and holding, the aux/emrg start switch should pick that same solenoid, which would give you a clue as to where the problem lies.
Now, just for clarification, the charging path is not the same path that the inverter uses to draw current from the batteries (unless it's wired incorrectly). You don't need a solenoid with much more than 150 amp continuous contact capability. Do make sure that the solenoid coil is rated for continuous duty, and I prefer using a 14volt rated coil.
If you locate the BCC box and need further troubleshooting documentation, just PM me. Good luck
