Oil filler Cap

I don't think there's been a vehicle made that had an oil fill without a cap, at least in my lifetime. It should be pretty evident from the fill tube whether there's some sort of threading or bayonet fitting prongs or whatever to secure a cap to it.

Your best bet might be to drive it to a decently stocked auto parts store and ask them to find an appropriate one to fit (or let you try out various ones to find one). You could also try calling Holiday Rambler and asking them for information.