When my onboard inverter/charger developed issues & stopped charging I had to find a workaround till I got it fixed. Bought a portable charger with a selector for flooded, AGM or Gel Cell battery type. I have a 4D AGM. Charge rate is selectable 2amp or 15 amp. 15amp is not very high but means that it takes longer to reach the maintenance stage.
If you are storing & have access to the rig there really is no need to get a portable charger. Simply connect the battery bank once a month, let the onboard charger top off the bank & disconnect for another month rendering the trailer electronically dead again.
