charge 12v 12ah battery for 120v house and 12v car system

I have created a ham radio/computer gobox for emergency use when bad weather hits.

I can go into areas with out power and communications and provide ham radio and packet service for people in need. The go box I have has 2 radios 2m/70cm and a low power cw rig. It also has a low power computer and four 5v 1A powered usb ports. I have two 12ah batteries in the go box mainly for backup and temp use. in my car I have a extra 12v deep cycle battery in parallel with the car battery.I have this battery switched so I don't strand myself by running the car battery down.

How can i charge the two 12ah batteries from the cars 12v system and from 120v house/generator power not at the same time of coarse? I need to do it as cheap and safe as possible and in side of my gobox so it all is self contained. I am looking for a circuit that I can build to do this.

Thanks for your help

For charging from the car you can use an RV Battery Isolator. For charging from 120V you just need a standard car battery charger.

Those are both a lot easier to buy then to build.

I need to add something to my post I will need to run off of the 12v or 120v power at the same time or while I am charging and I would like to contain this in my gobox so that it is simple to use. Basically I want to be able to run the gobox from 12v car, 12v internal or 120v and keep everything in the gobox's small space so that anyone can grab it plug it up if there is a place to plug it and communicate with the world.

You should still be able to do that if the 120v battery charger has sufficient current capability to power the radios. Just power the radios from the internal battery with the charger always connected. When the charger is plugged in it will charge the batteries and/or also operate the radios. Buy a high quality regulated charger for this purpose