Roof Vent Cap

I paid $7 for one at an RV store. It looked the same as my one remaining one and was the same size. But it doesn't fit. It has four little posts and a springy metal thing that is supposed to hold it on the pipe but it is just too loose. Mine has four posts with slots in them that catch the partial top on the vent when the cap is rotated. I might try cutting slots in the posts on the new one. Meanwhile, I have gone a year without the cap on the grey tank which doesn't seem to be a problem. What is the purpose of the cap, anyway?

I saw one vent on eBay that opened to the rear and might help the air flow when driving. That might be useful.