4 coolers sure is a lot of coolers.
I can see a defective one, or I can see one that wore out, and I can even see the remote chance of the 2nd one being defective but I can't see 4. If they were all bad you'd be hearing screams from all over the web from other RVers too. I'm thinking that there has to be something that's causing them to fail. Being a simple enough system there aren't too many things that could affect it. My guesses?
How about transmission pressure. The fluid pressure is regulated in ther Allison. If it's too high, the extra pressure may tend to "pop" things.
Is the coolant fresh? Have the SCA (PH) levels been checked? If the coolant is heavily acidic it may be eating at the cooler tubing causing it to corode and weaken.
In the same manner, is the tranny fluid clean and no contamination found in it? If anything acidic is in the fluid it could have the same effect.
Other than that, I can't think of anything else that could posibly affect it and cause pinholes to appear.