Why do they claim that the batteries became "toast" in a couple years or less? (Could be the charger fried them?) I'd get the charger or confirm the changes were made to make it compatible with the gel cells.

Then I'd verify that you have the batteries you think. You should have battery boxes and in vented location (not the engine compartment) for anything BUT AGM's or maybe Gel Cells. Even the Gel Cells or AGM's should be in a cool, ventilated place because they will outgas a bit. (A Hinckley burned to the waterline years ago because they upgraded the batteries to Gel Cells in non ventilated compartment. It doesn't need much, but hydrogen...

As for the Interstate, they market (maybe make) both Interstate and Optimia marine batteries. Some are lead-acid, some AGM, and some SC (sealed case) batteries. The Sealed Case may just be lead acid with NO FILL's like an automobile battery. I'm NOT SURE that is a Gel Cell. The numbering system is below:

http://www.interstatebatteries.com/cs_estore/content/product_info/marine_m.asp

You can also see the spec information. by clicking around the website.

In my opinion, a deepcycle marine battery that is lead acid SHOULD HAVE fill caps. Obviously if they do, then you know.

If the dealer says they were replaced, ask to see the papers for model and serial numbers -- if for no other reason, for warranty purposes.