Hi Abilenemike,
Welcome to the Heartland Owners Forum.
I assume you're hoping for warranty coverage on the repair. If you're the original owner and still within the 12 month warranty, then it's a question of getting to agreement that the problem is due to a manufacturing defect.
If you're the 2nd owner or if the warranty has expired, you may not get any help on the repair even if due to a defect.
But there's also a question of whether the tech has it right that the problem was caused by the slide being out of adjustment. Did he explain more precisely what he believes was happening that would cause a crack?
Sidewall cracks can be due to a problem in the side wall framing. If you have an aluminum wall frame, and a weld broke, that could account for the crack. And in that case, it's possible that Heartland might fix it as a goodwill accommodation. So I'd suggest you have a more in-depth discussion with the tech about the cause.
Another consideration is that if you have to pay for the repair, you should have an up front discussion with the Service Manager regarding their policy on warranting their work. In other words, if they patch the fiberglass and it cracks again in 3 or 4 months, will they want to charge you a 2nd time, or will they stand behind their work. Whatever they promise should be in writing.