You could try an alternative procedure for replacing the 4D's that worked well when my 4D's failed the first time at Cape May, NJ. The dockmaster at Utches Marina had a young fellow pick up a 4D and carry it up to the shop, and bring back a new one, then did the same with the other 4D, no charge. Great service!
The flex panels are a good option. I installed one on top of my RV and it works very well. I think you will want to keep them in place, though. The controller that you will install for solar power will be a smart device, and will charge the batts through all the proper stages of charging, and maintain a proper float charge. The old charger on the boat is a dumb charger and will not do a good job of charging and maintaining the batteries. I usually leave the boat charger off when I am on shore power, and let the solar controller do it's thing.