and there's good, and rarely can you get both in one.
In your case your want a charger
with three separate outputs, because each charging battery
is "one bank" and the house bank is a third "bank". Each bank needs to be charged and regulated separately.
So, unless you get rid of one starting battery (or the two are already a matched set in parallel) you are already restricted to chargers with three separate outputs and regulators--more money
Then you need to look at the power rating (amps or watts will do) for each output on the charger. Rashly assuming that you will never want to recharge the batteries faster than "overnight", you can divide the amp-hour capacity of each battery bank by ten, and then make sure each output from the charger can provide that many amps.
If you are more conservative, and you never discharge your batteries below the 50% point, you can cut that power rating in half and still fully recharge overnight. There's room for some personal thought about how much power you are willing to buy--and how fast you will be able to recharge as a result.
Wet cells generally can't take a full charge in less than five hours, so there's also a point beyond which you can't use more power. And if your charger has an option to "equalize" the batteries once a month--you'll see more months, or years, or performance from them as well. That's a feature worth looking for, it will pay for itself.
One of the members did have a new charger catch fire shortly after installation--so do take precautions, fuse it, consider placing some sheet metal on the bulkhead behind and above it, and make sure to stick around the first few times you use it.
Just in case.