This is something I recall and to some extent have tested in a limited way.
Deep cycles as a general thing (at least flooded ones that are true deep cycle) do not take a fast charge well, any more than the can deliver a fast discharge as output to start a car. The newer marine/deep cycles may not be as slow to take a charge because they are designed to deliver a starting charge. IDK for sure.
Had to do with the difference in the plate surface area of a deep cycle limiting the rate at which it could take a charge.
As for my testing it was a small gas engine hooked to spare alternator from school bus with a regulator. Using a larger pulley on the engine I could increase the rpm's of the alternator to crank out more amps. But had to run the engine at 3/4 throttle to have enough power. Guy at battery
store said that was fine for jump starting but for charging a deep cycle I should try smaller pulley, lower alternator rpm's and run the engine at lower throttle. Tried it and it seemed to do a better job putting a charge on the battery with less noise and fuel used. Took awhile to get that charge however.
Me thinks a modern smart charger would do the best job of charging the battery efficiently using a generator. Using solar
I would think a good charge controller would do the best job.