You're going to have a problem with your wood. Whatever you do it is likely to fail. No film type coating, i.e. varnish, lacquer, paint, urethane, even epoxy will hold up on wood, especially oak, exposed to the weather. Wood expands and contracts from temperature and moisture changes more than any film finish can accommodate, so it will soon develop fine cracks (called checking) that will allow moisture to penetrate and migrate under the film lifting it (peeling). There isn't anything you can do to stop this from happening except to put the truck inside a climate controlled enclosure, or not ever put a film coating on the wood. A fine grained wood will take longer to lift the film, but it will happen. Oak is an course open grained wood that moves more than most hardwoods, and a film finish will fail within a year. Oak also has a large amount of tannin in the wood that turns black when exposed to moisture.
Adding stains only aggravates the situation, the solvents and pigments in the commonly available DIY stains are meant for interior work reduces the film finish adherence and accelerating the checking and peeling. Staining a wood to try to make it look like another always just makes it ugly wood. Any chips or dings will show a different color making them very apparent.
First, if you don't like the looks of oak, but want a natural wood look, change to a wood you do like. Second if you don't want the finish to check and peel, don't use a film finish. If you want a clear finish, use an anaerobic polymerizing penetrating oil finish such as pure tung oil (NOT "tung oil finish" which is just thinned urethane varnish with just a very small amount of tung oil so it can be used in the name) or boiled linseed oil. The tung oil is more UV resistant and durable. Third, if you don't want to have to periodically maintain the finish, don't apply one. Maintaining an oil finish is the easiest, just reapply more oil when the finish starts to look dry. A film finish must have all traces of the old finish and any discoloration removed, usually requiring the removal of a layer of wood by extensive sanding, scraping or planing then a new finish applied. A few tropical woods like wenge, ipe, and old growth teak are high in natural oils and have a tight grain, so they weather well in their natural state, but they will eventually all grey from UV exposure.

 

I'm a retired contractor and former blue water sailor so I do know a bit about finishing wood. The marine industry has some great products for finishing wood that does not involve using any type of film finish. I used a tinted rub on oil finish on all the teak on our Hans Christian 38. It held up quite well even when we were in the tropics...if the teak started looking bad we'd just rub on some more and blend it in. I even used it on an outside oak 3 tier plant stand I built for my wife when we lived in Washington state. Held up just fine on the plant stand up there even with all the rain.
West Marine has product called WOOD BLEECH, for all woods that will remove on the embedded oils, stains, etc from any wood. I've used it and it works great...time consuming to use but I'm retired so I've the time

So that is what I'm planning upon using on the bed wood...don't really want to spend $500+/- for new wood...maybe if I was doing a garage queen.

 

Thank you, that's exactly the finish I suggested above less the tint (tint and stain are two different animals. Most stains are solid pigments (basically paint) in a solvent carrier that is meant to change the basic color of the surface of the wood that is applied before a finish. Tints are transparent dyes or toners dissolved in the finish that is meant to slightly darken (age or "antique") the tone of the finish.