Rechargeable Power Packs

I have installed a large box in place of the plastic battery/propane box on my 320 in anticipation of installing two 6v batteries. We then started looking at the portable rechargeable power packs such as the Yeti 500Wh portable power station and the EcoFlow River Pro.

We were at REI today looking at them and generally this is what we heard. You can charge them off of your truck, there is a cigarette lighter adapter, 120 v when you are at a site with electricity or solar (we have a Zamp and I'm not sure whether  we would need another adapter).

The output seems to be the problem. I had expected that once charged we could connect it in series with our 12v battery and it would just add power to the 12v battery. The sales person wasn't sure that could be done. He thought it had to be kept independent from the 12v in the T@B. If we wanted to use it we would have to say plug the TV directly into the power station or plug the refrigerator into the power station or plug the water pump into the power station.

The power station does have AC output so that's nice for a toaster or something else that needs AC.

Has anyone researched this to determine if these units can be connected directly into the 12v system of the T@B and just add power to the existing 12v battery?

Thanks

Gene