battery storage question

[masonjs at nrtco.net]

I was just charing my bike battery for winter storage.  Now due to 
also having, boats, ATV, lawn tractors and the bikes I have 6 summer 
use batteries.  All the summer use vehicles are stored in buildings 
without power so I pull all the batteries.
I used to charge them every 5 -6 weeks in the winter.  Now I go south 
for 2.5 months.  I was just wondering if a single battery tender or 
one amp charger would do the lot of them if I just connect them all in 
parallel.

Jim