Discharging a lead-acid battery by any amount reduces its cycle lifetime, but 50% isn't any sort of magic number. In fact, for the vast majority of campers, the batteries die because of abuse or their calendar lifetime, not the cycle lifetime. Getting 500 cycles instead of 1000 simply doesn't matter - it's the difference between dry camping every weekend of the year for 5 years vs 10 years, and the vast majority of people simply aren't off-grid that much. Discharging by 80 or 90% a few times a year simply won't make a difference, as long as they're recharged soon, and it's ultimately cheaper than buying more batteries to meet some "50% rule." The 50% thing comes from off-grid daily use, where it does make a difference.
The other thing which kills them is not recharging them shortly after discharge, or not recharging them fully. The latter happens with the cheap converter/chargers most campers come with. Even the $20 from China eBay solar controllers do better in most cases.
