We purchased an ADCO Universal Fit Designer Series TyvekŪ Plus Wind RV Cover directly from ADCO for our 195RB. While Camping World has a sizing tool it is generic and assumes you know your trailer dimensions. ADCO has a sizing tool that you can use to select your trailer make and model. The Universal Fit options are designed to fit a range of trailer sizes and the overall dimensions of the cover are listed and they are designed to take up the slack as needed.
If you want a custom fit there are detailed instructions on the ADCO site that tell you exactly how to measure your trailer in order to get the best fit. For me the Universal fit works just fine.
ADCO site is here --> ADCO RV Covers, Travel Trailer Covers, ADCO Products
Navigate to RV Covers, Universal, Travel Trailers and the sizing tool will be at the top of the page. I checked and for Jayco you'll need to select year 2015 - looks like they don't yet have very many manufacturers listed for 2016.
If you go with a Universal Fit once you have the item # or the sizing information then you can Google for the best deal.