You don't really need a Trimetric, but it would be a "nice to have" for sure. Read the installation instructions for the Trimetric on line (Bogart) which will answer some of your questions.
Do check those panels if they are 33 cell or 36 cell. Makes a difference in what your solar can do. ( In 1996, Siemens made both size panels)
You might have a failure in the ASC. You can check for that in its manual, which is also on line (Specialty concepts) They have an excellent manual.
The sudden drop in battery voltage might be from a load you didn't know was on, such as leaving the inverter on running something? Or if whole house, the converter left on with the inverter--draining the battery?
You mostly need a digital multimeter to cross-check what the systems monitor is saying, and for many other RV related measurements.
