2001 Beaver Patriot Thunder fresh water tank overflow

Well figured it out, yes indeed its is the overflow. I was wrong on the cap being on it, darn old eyes, I had the wife look up there and she noted it wasn't a cap but a dirty screen. So I went ahead and filled the tank until it overflowed and sure enough, the water flowed out like it should. So learned another thing about the coach, that's for all the replies.