Not to be the bearer of bad news, but it may not be your cap. Your model coach has had issues with cracking the radiator and causing the wiffs of antifreeze. Monaco rigidly mounted the radiator with the charge air cooler atop it. This caused he radiator to eventually crack. I smelled coolant for almost two years on my Diplomat and just could never find the leak. Finally, I located it. They often start leaking in the upper left or right corner of the radiator. Since it's at the top, it doesn't just run out. These coaches don't make a lot of pressure (cap) so they don't force a lot of coolant out when there is a leak.
Check your radiator carefully. Back when mine went bad, I lucked out and was able to buy a brand new one from a surplus company that bought a lot of Monaco stock when they went out of business. The newer version was an inch shorter and had rubber spacers between the radiator and CAC, eliminating some of the rigid mounting.
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab