Charging problem... 2009 Navion iQ (Sprinter 3.0 V6).

[MyGplante]

Hi, I'm having trouble diagnosing the charging problem on my 2009 Navion iQ (Sprinter 3.0 V6). While driving on the highway, the RV batteries (2 x 6 volts Trojan T-105) started to drain slowly. I have a display on my iPhone that allows me to see the RV's Victron Shunt in real time. It's as if the alternator can't supply enough current to compensate for the electricity demand (air conditioning fan and Sprinter compressor plus RV fridge). Everything worked fine before. The motor's 12-volt gel battery is about 5 years old, and I've been having a bit of trouble recharging it lately.
If the Sprinter is stationary and the engine is running, the alternator's charge reading is 13.2 volts. The Sprinter's battery is holding at 12.8 or 12.9 volts. The Cole Hersee 24213 solenoid (200A) seems to work fine, it clicks on start-up but is it enough to check? In short, I don't know whether I have an alternator problem (180A OEM and 14.4 volts normally) or a problem with the engine's 12-volt gel battery (4 years old) or something else.

On a 5-hour highway trip, I got down to 12.3 volts everywhere (RV batteries and motor battery).

1- When I'm driving and everything's off, I get 40 watts going to the RV batteries (photo 1) for a while, then it gets worse.
2- When I turn on the RV's fan speed, air-conditioning compressor and 12-volt fridge, I'm down to -28 watts, and it can go as low as -80 watts (photo 2).
3- Photo 3 shows the Sprinter's battery Volts on my Garmin GPS ODB2 connector.

What can I do to find my trouble (12-volt engine gel battery, alternator or Cole Hersee 24213 200A solenoid)?

Thank you for your help. ;-)