another old battery question

Three times in two years my cranking battery has failed me.I fully charge it
,have it checked at a "battery terminal" and three outings later the thing
will barely turn it over , much less start it.I know the thing to do is to
charge it every time I charge my trolling batteries , but I always figured
that was what the alternator was for.Do you all charge your cranking
battery every time you hook up to charge your trolling batteries.? Would
you guess a bad battery or faulty alternator?? Or just someone lazy on the
maintaince end???
Thanks
Ken