Need advice on adding aftermarket battery charger/maintainer

[Bocephus]

I have decided to add an external battery charger/maintainer on to my toy hauler. I want a simple 120vAC module. After researching I have learned that a 3 phase charger is the only way to go. However, I am unclear on a few things yet....

I will disable the battery charging circuit on my camper's converter while I am charging with my add on charger........BUT, can I leave all the battery cables hooked up to the camper battery while I am hooked up to the aftermarket charger?

What size charger will be sufficient? I dont want something that is used to maintain the battery on a bike or atv in off seasons i want something i can plug in and have it charged in 24hrs or less IF POSSIBLE. I have two 6v batteries if that makes a difference...

I want something I can leave hooked up to the batteries all the time. I want to be able to open the door, plug in, and be done. Ive read that some cheaper chargers will actually discharge the the 12v battery if it is not plugged into 120vAC. I obviously don't want this either

[KyDan]

I have one of these--
Amazon.com: Black & Decker VEC1095ABD Smart Battery 25/10/2 Amp Battery Charger: Automotive
It's very light weight since it is an electronic charger vs old school transformer type
charger. $60 with free shipping from Amazon.
There are mixed reviews on Amazon. Some folks found it to be short lived but others
loved it. I've had no problems with mine and it's 4 years old.
I don't use it all the time but when I need a boost I've got it.
I use it with my Honda eu2000 to quick charge my trailer batteries on occasion.

If you aren't in quite that big a hurry I recommend the battery tender plus from
these guys-- Battery Tender Plus
It's $40 bucks with free shipping.
I know you said you wanted a full charge in 24 hours or less but this one is
bullet proof reliable and won't harm your batteries.
It's a 1.25 amp fully automatic charger that you can leave connected long term.
It will give you a decent charge in 24 hours especially if your batteries
are not terribly discharged to begin with.