Yes, a solar panel on the roof is the best investment you can make, if your RV is in storage (outside) for any length of time.
I put a 100 watt panel on the roof and it keeps my 3 batteries (chassis and house) nicely charged between trips, and that sometimes is a month or longer.
I just check the batteries for fluid level from time to time, and run the generator and air conditioning every couple of weeks, but never have a problem with the batteries.
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