The G4 and the Tender are both adequate for battery layup in a fully charged state
So lets say someone has a 2 bank setup consisting of, say, 4 batteries total. 1 main cranking, 3 house. After spending 6 hours at the sand bar blasting the tunes, then a 15 minute ride back to the dock/ramp, I can assure you that 3 battery house bank is not at a fully charged state. Now what? One charger to recharge and then a maintainer for the winter? The proper charger of your brand choice, will do both. Like the NOCO Gen series if thats the brand you are partial to.

Cant comment on "the tender" without knowing a specific model. But to say " they have one purpose and that is to keep a small charge on the battery and they won't overcharge and dry out the battery" seems like a contradiction to me.

Also, promariner makes a number of models. Since no specific model is named here, maybe not paint them all with the same brush.