I need your help with Ford Carryall

[JeffGraves]

I have a 1984 Ford Carryall 26 ft. I have a few questions about it, as this is my first r.v. and I've only had it a few months. First, in addition to the two gas tanks on the driver's side, there is a line on the passenger side with a cap that doesn't have any locks or door or anything. It's just there on the passenger side between the passenger door and the side entrance door. It smells let me gas when I bend down and smell it. What is it for?
Second, my tires are shot. I need to get some tires on it before the season begins. What size rims and tires? The specs say I have 16.5 inch wheels. Can I run 16 or 17 inch tires? I'm just trying to get tires on it without spending any more than I have to.
Lastly, how do I go about u winterization? I don't know where to add water, how to get shower or toilet to work or anything. The r.v. came from a trustworthy source, so I know everything works. I just don't know how to operate anything.

[Unplanned Tourist]

I would suggest that you run it into an RV dealer or repair shop and have them go through it with you. They can de-winterize it for you, and also give you tips on what does what.
As far as tires go, if it has 16.5's you have to stick with them.
Happy Glamping.