Hi Dan. I assume by "redwood pine" you are not referring to
Sequoia sempervirens, also known as "Coastal Redwood" or "California Redwood". This species of softwood would be an excellent choice due to it's close, clear grain structure and it is almost impervious to rot and bugs.
The "pine" I am familiar with here in the US is
very soft, somewhat pitchy, usually filled with knots, and doesn't stand up to weather very well. It's also difficult to protect with top coatings (varnish, paint) due to the oily nature of the grain. It is also very difficult to seat fasteners into securely.
Oak would not be my choice for an exterior hardwood. It has very porous, open grain which would tend to wick up moisture. Exposed Oak actually rots fairly quickly. It's also somewhat cantankerous to work with, as the grain is large and frequently contains faults.
When I built the skylights for the Housetruck, I chose Philippine Mahogany as a "middle ground" hardwood. It has close grain, is available in clear (knot-free) lengths. It is easy to work and takes fasteners well. While it is susceptible to moisture rot and wood boring organisms, it takes preservatives well. Lots of brightwork on boats is made of mahogany.
Of course, if you are looking for the ultimate in exterior wood,
teak would be the best choice. It is very hard, but close grained and has an oily consistency, so much so that preservatives are only needed as an appearance enhancement.
Teak sounds expensive, but in many parts of the world, it's a "throw away" species. Crating and freight dunnage is made from less choice bolts of teak. You can frequently find scrap timbers of teak around boat yards and shipping ports, it's used to secure shipments from moving during travel.
Whatever you use, you probably won't be needing very much of it to make a few windows, so if you have to purchase it, it won't be especially dear. The biggest expenditure is your time to construct, finish and install the items, and making sure you only have to do it once is important.
[edit] After blabbing on a great length about building your windows, I posted and then went back and re-read the original post, and now understand that you are looking for wood to
frame your windows. Duh. Well, I'd be hard pressed to recommend going all-out with hardwood in this application.
What do carpenters over there use to build exterior structures, stairs, flower boxes, retaining walls, etc? Over here we have a variety of outdoor woods all of which are chemically-treated
Douglas Fir or Hemlock Fir, referred to generically as "pressure treated" or by a variety of brand names like "Sun Wood".
If you don't want to install treated wood in your truck, consider something like the Doug Fir with some preservative on it. I used Fir to frame the Housetruck windows thirty five years ago and they aren't (quite) falling out yet.
I'd say your selection should be one that holds fasteners well, doesn't tend to crack when screwed into, and takes paint well. The Coastal Redwood still sounds good to me if you can find it. It's so common on the West Coast here that they build fences out of it... [/edit]
