Well, it would probably be best to use a wood like cedar or redwood (not very plentiful in your area), since it would have natural benefits that other soft-woods wouldn't have. Hardwood would probably add to much weight to the trailer, unless you were going with a veneer or very thin planking.
It would be a shame to do all the work, to get the beauty of wood, and then paint it. (just my opinion)......so I'd go with either a stain or clear-coat.
My guess (haven't done this before), would be that you would be stripping the sides off the RV. This has a benefit, in that you can use them as templates, if you are going with a veneer panel to replace it.
Question: Have you considered taking the TT to a body shop, and asking them what they think about repairs to the fiberglass? You may find that you can get away with a good sanding, then a painting, with a clear-coat.
May not be as "jazzy" as a wooden TT, but could probably be a whole lot easier, and cheaper (just how much to you really want to invest in this old TT???)
Just my $0.02:)
