I just measured it to be sure. It's actually 17' from the rear edge of the spare tire to the front of the fiberglass shell behind the propane tanks. Without including the spare tire, it's just under 16', so you suggestion will work. The spare tire has it's own cover. Thanks.
I bought a calmark for my 21. Fits well and after a full year of use it's still in good condition. I padded all the corners, TV antenna, vents with old yoga mats. I also put swim noodles between the awning and roof to smooth the transition. Anything that could cause rubbing or wear spots due to wind was padded.