In 68, the closest match in spray cans is Fusion Ivory White. Not exact but very close. Not sure how Fusion will last on metal but it's the closest. You can pretty much use any white if your repainting.
I am not too sure how the transition to the Coachmen era of the Shasta trailers affected the paint codes. They may have changed them up by the time the '76-s were coming out, but I found a Rustoleum color that came pretty close to the weathered white of my 1970. It is called "Canvas White", product number is 7789 (UPC code = 20066-77898), Gloss finish protective enamel, spray-can version.
I had found it on the shelf at our local hardware, however after they changed over to become the latest store swallowed up by Ace, they were not stocking it (or many other Rustoleum colors either...) but they were good enough to order me a 6-pk case. I'm not even certain it is not discontinued, which would be my luck, but I do still find it available through sellers on ebay.
I also purchased a quart can of the same color through a seller on ebay. I'm planning to paint the roof over again, but that may not happen for a couple summers. Tough to get a true representation of the shade of it on the camera, but here is a weak attempt...