So this kit has been in the truck for years and worked flawlessly until now, and I am still not sure if it's an issue with the kit itself or something else. But I'm stumped.

Starting battery (less than one year old) has all the electronic accessories tied to it. Second battary is completely isolated via a switched solenoid. Dual battery kit charges both during normal operations. My secondary battery is holding and maintaing a charge just fine. My primary battery however is not. It sits at 10v, just enough to start the truck but won't run my fridge at all with the vehicle off. This is the second battery I've had this issue with in the last couple of years.

Anybody have any ideas? All connections are tight.