Mismatched batteries

[femuse]

We need to buy a 2nd battery for our trailer. The geniuses who sold us the trailer, let both batteries freeze all winter. They gave us a "new" Interstate SRM-27. We just noticed the sticker with "date sold" on box as June 2002.
This battery must even be older than that. We can barely make out the stamping: in front of negative post, it may say B2I4. The B2 then should mean it was made in January 2002. Over a year old. Could have been used for all we know.

My questions is: if we buy a 2nd Interstate 27, are they going to be so mismatched that we are wasting our money on this second one?
Wouldn't we be as well to spend less (say deep cycle 27 from WalMart), wait a few years, then replace them both with 2 Interstate #29 ?
One way or the other, they are going to be mismatched. Any feed back on this mismatch problem ?

[Pick]

I have an Exide Group 27 "dual purpose" battery in my AS, that I got at Tractor Supply for $59. My truck has dual batteries, but they are cranking batteries, not deep cycle or dual purpose. When the trailer is connected to the truck, both batteries are connected together. I have seen no ill effects to either battery with this arrangement. Have gone 2 days without hookups using this arrangement.