Welcome to the forums.
As I have already dealt with these issues, here is what you need to know. First of all you are correct the Parallax charger is only 2 stage and will never condition or maintain your batteries correctly. If you dry camp a lot you must change it out for a good 3 stage. I went with the PDI 9200 series converter/charger charger.
I have been extremely happy with the PDI converter/charger and price wasn't all that bad.
2nd, If you have the Trimetric already you should follow the instructions for installation. You MUST use a shunt inline with the NEGATIVE battery cable that attaches to the frame. You can use standard telephone or Cat 5 twisted pair network cable to run from the shunt and battery to the meter. The meter records all amperage use coming and going to the battery bank and for that reason nothing should be directly connected to the negative battery cable between the shunt and the battery. Again follow their directions for installation.
I must say that I will never own another rig witout a Trimetric as it allows me to keep a very accurate eye on everything that happens with our battery bank while dry camping from solar charging, inverter power use, you name it.
Running the wires is of course different in every rig so you have to do what is more convienient. The Trimetric doesn't have to be with your other monitors, just somewhere easy to see and read it. I have found though that where there is a will there is a way. It took me several days of climbing under our rig, through compartments and into cabinets to figure out how to run our wiring. Study how other wires are run in your rig and take your time.