Pine, Poplar, and Oak are all good, though "Pine" in general conversation can mean any of a number of species, depending what part of the country you're from.
The best "Pine" I've come across is Southern yellow Pine.
It is used extensively for roof trusses, headers, and other weight bearing applications in home and industrial building...because of its strength.
It is heavier than your standard variety of Pine, but usually several times stronger.
Choose a piece(s) with perfectly straight grain, and rip out the ribs needed.
Poplar is slightly stronger than "Pine" generally more stable, and takes paint very well, though is difficult to use on a project that will be stained because of its wild variety of colors, the most prominant of which is green.
Oak is rather heavy, but also very strong.
Good use for Oak is the ribs that hold your hatch hinge.
As has been mentioned, some of these woods are very expensive, especially the ones that are sold as individual pieces wrapped in plastic.
They are obviously designed for the weekend project where the builder would rather pay for convenience...
Menards is a good example, you can buy the very expensive prewrapped pieces, or you can choose from their bin of rough edged, and odd length Oak boards...at a 75% savings.
All you have to do is cut them to size...
Though generally known in the building world, a couple things new builders might not...
Always pre drill!!
And when choosing/using lumber, leave the pieces with knots at the store.
It costs more for premium lumber that doesn't have knots, but a structural rib with a decent sized knot will snap like a match stick with only minor pressure applied.
Obviously not everyone can afford premium lumber, but if you're building a trailer, cutting large pieces of wood into smaller pieces of wood is part of the build, and a good way to get good lumber for a fair price.
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...
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