I put an R15 in the guesthouse a few weeks back -- and, now that my father-in-law is here, we ordered another to put in his 5th-wheeler.
Installer showed up with it, today -- and our measurements in advance meant nothing. Turns out I have one model of R15 -- and they brought another [R15-500] which turns out to be 2" deeper and overlaps the shelf we set aside in the trailer.
Bah, humbug. Carpentry for my wife's father -- instead of an easy install.