Connecting Solar Panel

I have a portable solar panel that I use to keep the coach batteries topped up when boondocking. With my fifth-wheel and two 6-volt batteries in series, hooking it up was obvious--positive on one battery, negative on the other. Now, with two 12-volts in parallel in the motorhome, I'm not sure. Same as with the the 6-volts (pos. of one, neg. of the other) or pos. and neg. of the same battery?