Offhand, I don't have any authoritative citations. But Googling it (something like short-term vacation home 27.5 or 39 years) will show you that MANY places say 39 years, including such websites as AIPCA, NATP, Kiplinger, Forbes and more. Although admittedly some were just generically taking about short-term rentals, and maybe not specifically referring to a single unit.
I didn't spent much time looking, but at first glance I couldn't find any website that was arguing for a 27.5 year Recovery Period.
