Are the batteries removed or disconnected while the trailer is stored or is power available 24/7/365?
Most owners of solar battery tenders have a very small panel, usually under 20 Watts. Those small modules may not keep batteries charged if there is any load on the batteries.
Really not enough information to make an exact diagnosis but, all things considered, it sounds like the batteries may have sulphated from not being charged correctly.
Has anyone checked electrolyte levels, put a meter on them, or done an equalization charge?
