If that's all you want to do then all you need is their cigarette lighter charging kit.
You can use that in both vehicles and be done with it.
However I think you will hate life if you deplete the Yeti and want to charge it from say, 10% to 100%, as it simply takes too long.
What I would do in...
I can't really speak to how well your alternator would charge a 2nd AGM battery. My understanding is that to charge a 2nd AGM battery properly on my 200 series you need a DCDC charger. So I have an Odyssey Extreme AGM as my 2nd battery charged with a Redarc BCDC 1225D. It works superbly. If I...