OK, I bought the 12 volt "plus" model. Now how do I use it ? If I hook it up to just the engine battery, will the house batteries get charged ?

I have used battery tenders since I purchased Barth I and continue to use them in Barth II. It is an easy way of keeping your batteries up to charge without having to plug in your coach. Mind you, this is just for maintaining your battery charge while sitting idle. You cannot use this, or any other type of trickle charger, to maintain your batteries while operating 12VDC items in your coach. It won't keep up.
You need two battery tenders to keep both your coach and chassis batteries charged. One will maintain the bank of two batteries, it just takes a little longer to bring them up to the maintained charge than if you were doing just one battery.
I have purchased a two bank battery tender and mounted it in the chassis battery compartment of my coach. I then ran a 12' extension lead from one of the tenders banks to the coach battery compartment. On both sets of batteries in my coach I attached one of the ring type battery leads and plugged in the tenders lead. While my coach is sitting in the yard, I just run and extension cord to the tender and then plug it into a standard 120V receptacle. Now while the coach is sitting it is keeping the batteries up to a charged level. AGAIN---this is just for maintaining a charged level in the batteries while it is sitting. It is not meant to replace plugging into shore power and using the converter to charge your coach batteries while using your coach! But I do not run or operate anything while the coach is in my yard. So I just want to make sure the batteries are up to snuff when I do take the coach out on a trip. If I have to use the coach for anything while in the yard, I use the generator for power (when it is working, but that is in a different post by me ...)
Another note on battery tenders/trickle chargers---they maintain a charge level in the batteries they are hooked up to. They are not meant to charge a dead battery. I also learned the hard way that they may show a battery being "charged" (a green light on the battery tender), but that is just telling you that what cells were able to be charged are charged. If you battery has a bad cell or two (which means it needs to be replaced), the tender will still show charged but the battery will NOT perform as it needs to. So though it may show your chassis batteries as being charged, do not get upset if your coach does not start. You still need to check out your batteries with a tester to confirm all the cells are 100%. But I have been using these battery tenders on my cycles, car collection and Barth's for years. They definitely help prolong the life of the battery. Most batteries today are good for 3-5 years, depending on your usage. By using battery tenders I have been able to get double that usage out of most of my batteries.

Thanks so much. I get it now, I think.