Some of those caps have a screw that needs to be removed. If there is a small hole on top filled with caulk, dig into that to find the screw head.
I have already had to replace 1 of my vent caps, and my unit is only 4 years old. The sun really can do a number on those puppies.
I found vent caps and the bases at a local RV dealer for a couple dollars each. I just replaced the cap, not the base.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263