Deep Cycle Battery in Pickup

[rScotty]

The voltage in the two batteries will equalize, then the voltage regulator will kick in and the two will send a single signal to the regulator and prevent any overcharging.

Yes, the combination of two batteries will send a single signal back to the regulator. As I see it, the result is more likely to be that the larger battery (or AGM in this case) gets undercharged than that the smaller one gets overcharged.
And I don't think the undercharge will be enough to cause problems.
Equalizing always loses some efficiency of the best component. That's the cost of equalization. In battery charging just like in life.
rScotty

 

[MossRoad]

Generally, there is a simple Ford style starter solenoid between the batteries. when you have the engine running, the solenoid is CLOSED, and activated, thus charging both batteries. when the engine is not running, the solenoid is open, and Not activated, which effectively separates the two batteries, and the secondary battery is isolated from the starter battery. This allows you to use the winch off only the secondary battery and keeps your starting battery intact. That way you can run the winch till the battery is completely dead and still be able to start your engine. 10-4 ?

I know how they work. I was asking the OP if he had a battery isolator installed since he said his batteries were in parallel.

 

[RancherGuy]

What kind of battery will you be you looking for?
I just replaced most of my batteries in everyvehicle including tractors and generator this year.

I think I'm going with Costco regular batteries - group 78 as I recall. Wife has the card, so I've got to coordinate with her. I am the original owner and hope to sell it within the next year, so don't need anything with an excessive warranty.