Linseed oil is not water resistent. Water goes right through it however, it does breath and the water will come back out of it. It has to be applied VERY thin and then, requires a day in the sun to "dry". It will take many days (sunny days) to get a decent finish. If applied too thick it will stay gooy for a LONG time. It IS, however, easy to repair. If this is for a show truck that will rarely see weather, it might be OK but rather than just plain double boiled linseed oil, you could try Birchwood Casey's "Tru-oil". It's linseed with driers added and is more user friendly and leaves a harder surface. A better choice would be Chambers oil which can be found here
http://www.flintlocks.com/ . Permalyn in another option
http://www.laurelmountainforge.com/products.htm These are gunstock finishes but would be suitable for a truck bed. Certainly more suitable than linseed IMO.