bilsheryl, don't think anyone is picking on your tarp sails. Actually, you should be lauded for your ingenuity! Will an RV hatch work... YEP! Sorry I'll be the only one to say it.
It's waterproof, and it'll hold up just fine. My brother owns a Newport 17 (great little boat), no worries, it ain't the sails slowing you down, I swear that boat ain't happy until it's blowing a gale! Him and I took her out in 24-30mph winds stood her up on her ear, and still couldn't bury the rail! I am told the next step is rolling it, but to me it seems that's pretty hard to do.
Free means you have lost nothing if something goes wrong. I am good with that!
That cabin is small/tight on these, but usable, so any air is good air, so an RV vent should do you just find. Obviously a boatyard used hatch is probably going to be better, but you also don't want to cut a 18x18 hole in that foredeck either, it's pretty structural to that boat.