(slightly O/T) charging multiple battery banks with solar

This post is about boats, but not sail boats, and it's about solar power electrical systems, so I hope it will be tolerated on this board. You folks have the most knowledge of self-contained and waterproof solar tech of any community I know of.

My friend has a cabin on a small lake, and he recently equipped it with solar panels. The cabin itself has 4 golf cart batteries in series-parallel (to power LED lighting, a water pump, and an inverter), and he also has 4 Group 27 deep cycle batteries that are used to power the electric outboard motors on his boats (this is where the boats come in!). Just to be clear, these batteries get taken out of the boats, hooked up to the charger, then put back in the boat when ready.

We're wondering the best way to split his solar power among these 5 banks. I did some quick googling and I couldn't find any solar charge controllers that have 5 bank outputs, and even the 4-bank ones couldn't handle the amperage such a system would require. If possible, he'd also like to be able to tie in a generator as a backup and have it capable of charging all 5 as well. I think he's willing to go down to 2 boat batteries, for a total of 3 banks charging simultaneously, if charging all of them at once proves impractical.

Thanks for your help, and I apologize if this strays too far from on-topic.