I actually have 3 ways to monitor my batteries:
>The solar controller.
>A Xantrex Link-10 now replaced by the Xantrex battery monitor (XBM).
>The Prosine 2000 inverter control panel. (I'm told the 2000 is no longer available)
I'm guessing the monitor they're talking about is either one for the inverter itself or something similar to the XBM...a lot of people ask for the XBM.
All are useful but I find the Xantrex is by far the most important and strongly suggest considering purchasing the XBM or similar. Being able to see amperage is a key feature. Other available options are also nice such as battery temperature which comes with my inverter but can also be installed on my link-10 and I would guess the XBM as well. The XBM also shows as a fuel gauge but a standard voltage gauge like on the solar controller can work when the battery is at rest.
I have two 4-D AGM batteries. The van came with one and I added a second. I'm happy I did but many people might not need so much power. There are pros and cons for having two batteries of any size as compared to one. FWIW my views can be found at this Link
. There are several post in the FAQ's in this sections that might help you as well.
I do wish SMB would develop some better method of mounting the batteries under the van so they can be lowered easier because what everybody is saying is true, it's a pain to drop them. A good 4-D is expensive which is another factor.
Good luck on your build.