The rot wasn't as bad in mine but the PO hadn't been paying attention to water entry - no silicone or butyl around the bolts and power cords that were run in through the ceiling and a window that leaked.
I didn't feel bad about ripping it all out since the interior was crappy to begin with but if it had been a nice interior I'd have been #$%^^& mad.
It's not the storage that did it, it was not paying attention and taking proper precautions. Thousands of people put RVs and seasonal dwellings in 'storage mode' every year and don't have problems.