The PD4655 is a good unit, even if 55 amps is a bit much for a single 12v battery. I'd only really be concerned with this if the battery is super hot when the RV is first plugged in. In such a situation, and if you have the 'remote pendant' you could force it to a lower voltage and less amperage will flow, keeping the battery cooler.
Super hot is 95F plus and 120F means stop charging.
100 watts will do much to keep a single 12v battery happy. especially with light electrical usage.
Keep in mind that most people who get solar, also eventually start using more electricity, and then want more solar.
Adding more solar later might be as much work again, and more expensive as just getting more than required now. Portable is great, if one can leave it out without worries, and faces it east to catch the mornign sun. Solar flat on the RV roof starts providing something at sunup, and will not easily grow legs and walk away, and provides something until late afternoon/ early evening.
If you are in need of a digital multimeter, I recommend getting a clamp on Ammeter, which is a full function digital multimeter, but it has a clamp one can put over a single wire and see how many amps are flowing through it. An Ammeter is likely the best learning tool there is, when one also knows the voltage. With the clampmeter you can see how many amps are actually flowing from the charging sources into the battery from Solar.alternator or PD4655, and how many amps are flowing out of the battery from your DC loads, such as laptop lights and such.
https://www.amazon.com/UNI-T-UT210E...?srs=6579490011&ie=UTF8&qid=1491069071&sr=8-1
Above is not a personal product recommendation, just an example. Make sure any unit chosen can do DC amperage. Cheaper than the above and they are AC only
With it you can figure out how charged the battery is, and how much you can use, and when recharging one can get an idea for how long it is going to take to complete the charge.
Basically the more charged the battery is, the less recharging amperage it can accept at a given voltage. Hopefully that voltage is in the mid 14's whenever less than 100% charged.
So your pd4655 will likely bring your 50% charged single 12v marine battery to 14.4v at 55 amps, in a few minutes, I'll guestimate 10 to 16 minutes. BUt 14.4v does not mean fully charged. When the amps flowing into your single 12v battery, at 14.5v, tapers to about 1 amp, then the battery is or nearly is, fully charged. The hydrometer at this point can say for sure.
But in the future the ammeter alone can be used to accurately estimate how charged the battery is when charging by how many amps it accepts at its current voltage.
