Brand recommendations: Progressive Industries 9200 line. This is a stand alone unit, 3 stages plus, in doing considerable research a few years ago I was unable to find a better converter for RV's with flooded wet cells.
What size: Figure out the 20 hour rating of your batteries, you may use the following list as a guide if you wish but be aware it's approximate. And multiply by 0.3 (30%)
Group 24 75 amp hours
Group 27 90-95
Group 29 100-105
Group 31 125-130
GC-2 pairs (These are six volt golf car batteries when you buy them, when paired in series they become one big 12 volt battery) 225-230
And the result is the size of the converter to buy
NOTE; a battery with a couple of shorted cells can look a lot like a bad converter depending on how the RV was wired.
My PDI 9180 With Wizard died on me today.. Usuall problem
It is a "Plug in" model (They make them in plug in and hard wired, the only difference is the plug in models come with a cord attached) and I'd accidentally pulled the plug.. Restored plug to outlet. .all good all of a sudden. Thankfully battery status monitor caught it.