Well, These units are only meant to keep the batteries toped off. at 3.2 watts thats only 3.2W/12V=.27 amps. Not enough power to run anything, but 2 or 3 of them should keep your battery up when you Rover is left alone a couple days. yes you should be able to plug a few in, but even 10 of them is only 2.7 amps, you would not be able to run much off of them.
As far as charging the second battery it depends on how your batteries are tied together. If they are just put in parallel together then it would charge them at the same time.
Don't forget to
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first then ask questions later! The loose nut behind the wheel