How can I safely charge multiple deep cycle batteries simultaneously?
The "right" way to charge 4 batteries simultaneously would be to buy 4 small "trickle charger" style battery chargers and charge each battery independently.  This would allow you to safely leave all of the batteries that aren't in active use to charge at their own ideal rate.  This is the approach that your distributor is recommending to you.

It is possible, though far from ideal, to charge several 12V batteries (monoblocks with 6 cells) in parallel.  It works much better if they are all the same age and they must be the same brand and same capacity.  What you really want is for them to have the same specific gravity in each cell in each monoblock.  It sounds like you are using flooded batteries, so checking for proper electrolyte levels often will be very important.

I'd recommend the following guidelines for charging multiple lead acid batteries in parallel:

1) Ensure that the lead lengths between the charger and each cell are close to identical.  You would not, for example, want to connect the charger directly to the posts of battery A, then connect battery B to the posts of battery A, etc.)  Each length of wire will have a voltage drop associated with it while passing current.  You want to ensure that, under charge, the terminal voltages of all batteries (measured at the post) are very close to the same.

2)  Add current limiting (like a fuse or fusable link) in each connection (both sides) to each battery.  This will provide a basic (not comprehensive!) level of protection against accidental reverse polarity connection). 

3)  Use a good quality low rate (10-20A) charger

Your copper pipe approach satisfies requirement #1.  If you use leads with inline fuses (buy fuse holders from an auto parts store and splice them inline) and ensure that the pipes are mounted in such a way that nothing metallic could reasonably short the two pipes, you've got #2.

As for why your most recent experiment didn't work, it's tough to say without more information.  Wheeled automototive battery chargers typically have zero regulation - they are simply a multi-tap transformer (different charge rates) and a rectifier.  They are made primarily for jump-starting/battery assist type of applications and aren't a great fit for your application.  Research good quality trickle/maintenance chargers, and select one with a 10-20A output (selectable).  (#3) You can then more comfortably experiment with the charge rate that best matches your application.

Always wear eye protection when working around batteries.