Desulfation mode may be a trick. But
A progressive Dynaics 9200 is a fantastic charger (This is a converter) 3 stage plus. It does auto-equalization for a few minutes every so many hours (20 I think) and can be left connected to the batteries for a long long (years) time.
I have a 9180 with optional charge wizard (Equal to the 9200 which has the wiz built in)
When it failed, on a Sunday (Well it failed on Saturday) Getting a replacement was out of the question for at least 24 hours (Turned out to be 60 hours) so I hooked up a DELTRAN Battery Tender
Now this is a 1.5 amp battery tender just like the little ones you get a Wal-mart or ______. but it's also a 20 amp "Boost charger" and that's the mode I choose 20 amps for 24 hours and the batteries read CHARGED..
And a 70 Amp Jump Starter (have not used that mode.. yet)
About 100 bucks at Auto Zonew. but it sounds like about what you want
Use the 20 amp mode rto do rhew inirial charge thewn go ro maintainer mode for as long as you like.