THere are three different threads on the Forum at this time about maintaining batteries while the RV is in storage. There are different opinions about the care and maintenance of batteries, some based on use, some based on science, some on a combination of the two, and some with no real qualifications other than "it's worked for me".
I don't have a horse in this race since I have adequate solar to keep my batteries in great shape. Some good information can be found at
Battery University or at
The 12 volt side of life.
The bottom line is, that when a battery sits unattended, it is discharging and starts to sulphate. When a small current is connected to the battery that is greater than the potential energy inside the battery, it is being charged. A user will decide towards which side of this situation is applicable to their use and how much loss of battery capacity/longevity is acceptable for their use. Logistics may come into play as not everyone has power close by their RV for the purpose of charging. The worst situation is leaving the RV battery connected with the parasitic loads common to most RV's drawing the battery down to unacceptable levels. A cheap marine hybrid battery won't endure much of that before it no longer works.
Those 1.5 watt solar maintainers will probably not keep an average sized battery from discharging.