There are two separate aspects to this business.
The OP didn't say whether he wants faster charging but did say his pair of 6s were not getting fully charged. I ran through my "learner" set of 6s when finding out that the single stage 13.8v converter was not getting them fully charged no matter how long they were on with shore power. They got sulfated beyond recovery.
You need the high voltage as shown in the above link, but the amps don't matter so much-- if you are on shore power, it will get done.
People here report that their 6s do quite well on converters that only go to 14.4v. But those don't do the last part of the recharge at 14.4, they do it at 13.6. I cannot confirm from own experience if that is true, but I doubt those folks are lying! Maybe it just means the batts will last longer than at 13.8 and that is long enough for them, even if they would last even longer if they saw 15. I don't know.
So you can possibly solve the main problem of getting a full charge on shore power by just using a higher voltage charger/converter. That will help but not necessarily solve the second problem of fast charging off the generator when doing "50-90s." For that you also need a high charging rate (charging amps wrt bank capacity in AH)
