I made the mistake of getting the Adco 52239. Its a great product but since the trailer was in storage and I couldn't measure it, I erred on the side of caution and ordered the 18' 1" to 20' cover.
It fits like a glove. A giant, oversized, flapping in the wind, glove.
2014 Jayco Jayflight Swift SLX 184BH Baja
2016 Toyota Tundra DC 5.7L
Nights Camped in 2015 - 24
Nights Camped in 2016 - 9