http://www.amazon.com/Black-Decker-VEC1093DBD-Battery-Charger/dp/B000EJQJ1G
http://www.batterychargers.com/ProductDetail.aspx?ProductName=940261042
The black and decker was sold under another name and was a highly sought after charger. I discovered that the charger company was bought by black and decker and was being produced by them or at least distributed by them. I bought it and it is everything the boating blogs said it is.
I used these two chargers to maintain my original OEM batts (interstate RV/Marine starting battery). But not my industrial RE house batteries. But for the Marine/RV deep cycle starting batteries that came with our rigs, these will do a good/great job for you. The black and decker will charge your battery to its max. It is a three stage, and it actually has equalize function as well. But you have to stick a paperclip in a small hole to activate it. It also has a battery save feature where it will try to save your battery if possible.
The Black and Decker can even save a battery. I know this because I was able to save a tractor battery that was near death. I'm still using the battery that I thought was junk. I know that the battery was completely dead for over a year, I was just saving it for a core. I had another tractor battery that it could not save but it was far gone.
The SP2 is a maintainer. It too is a smart charger as well. It will only provide the current necessary to maintain your battery. It is made to operate for weeks at a time. It is not a good charger, but is a good maintainer. Its automatic. It runs through an algorithm and takes a long time to charge but can be left alone to maintain.
Personally, I would buy the black and decker, charge your batts over night, then disconnect it the next day. Then I would hook it up the next time i had the opportunity. Mine generally only takes 30 minutes every 30 days. It takes 30 minutes because it goes through a programed charge algorithm. So once it decides what charge is needed it has to go through a sequence. If your batts are significantly discharged, just leave it overnight.
Your disconnected batts will only discharge a small amount per month. If they won't hold a charge then your wasting your time trying to charge them. Get a new set and be sure they are charged before you leave the dealer.
If you cannot visit your fiver every month then you need the SP2. leave it hooked up. I do and it has worked flawlessly.
Do not use these batteries on large true deep cycle house batteries such as the trojan or continental or the other industrial type deep cycle batteries.
In comparison to your onboard charger, these are light weight. But your onboard charger has a different task. It is designed to bring your batts from 50% to 90% quickly, then go through a process of absorption and float. And it is designed to use everyday. The black and decker can perform the same function but it cannot cram 150 amp hours into a battery bank in 2 hours. And it is not designed to operate every day as is needed by your fiver when in use.
Remember between 12.77 and 13.1 v is a (nominally) fully charged battery. 12.4v is 50% charged. 12v is discharged.
I hope you find this information helpful.