I am looking at 2001 ranger 518vx.. It has a two bank onboard charger for trolling motor batteries but not for the cranking battery. He says the cranking battery recharges thru the alternator so you wouldn't need a three bank charger. Is this true? You never need to charge the cranking battery?

He is right you do not "Need" it but it is nice to have so you can be sure all your batteries are at 100% when you head out.
This is what I use it works great and works with wet or gel batteries and can usually be had for around $100.
http://www.pmariner.com/productFeature.php?ProductNum=31313

If you fish a lot or put a heavy demand on your batteries you might want to look at something with higher amp output but for someone like me it has never let me down

With todays electronics and electrical demands of your boat you defintely need to charge you cranking battery regularly.

And unless you make long runs frequently you are not charging the cranking battery very much

all charging systems are to maintain battery

I run a 3 bank 15amp output, better to be safe than sorry.

I have a 24v trolling motor and a two bank charger. I just bought a battery charger from Walmart for around $50 that I use to charge my starting battery about once a week or if run the battery down using the livewells or other electronics all day. No big deal and have never had an issue with the starter battery yet.