Batteries Not Charging

Yes, corrosion anywhere along the input path (from the converter) could cause what you're seeing. A point of corrosion in that 5 foot input path all the way from the converter's terminals to the lug on the input side of the solenoid could be the cause. If so, it's a corrosion point outside of the solenoid, so a new solenoid is not required.

Also - it may not be corrosion in the 5 foot positive input path between the converter and the battery(ies) ... it very likely could be corrosion in the negative battery path ... maybe where the negative cable from the battery(ies) connects to the frame of the RV.