I would just use regular grade plywood and top coat it with marine grade varnish (spar varnish) or some type of good epoxy sealer. If you do buy the $25 plywood put the sealed side up. There is an aluminum skin under the wood floor that protects it from road moisture.
I wouldn't take the interior panels out to paint them, way to much work for little or no gain. Do a search of the forums for interior painting, I think you'll find a lot of good information to get you started.
In order to get ready for your vacation I would concentrate on the mechanicals to make sure the motorhome is safe to drive. Brakes and steering are the two most important items on your motorhome. Without them being up to snuff you'll find driving the motorhome an unpleasant experience. Cosmetics and other repairs can be made when you have the time and money to do so. This is the approach we took on our 310 when we bought it and while its not necessarily pleasing to the eye its still a lot of fun and above all it SAFE.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)