Ours is real tight with the sheets and comforter on it. I really have to give it a hard push to get the pins in. What I mean is, when the mattress edge makes contact with the roof of the slide I still have about 4" to 5" to go till the pin lines up. Dont be afraid to get a little "mean" with it, all its gonna do is squish the mattress and the soft stuff.
You should travel with it down, so with the small amount of time it is up I dont mind it being tight. you can always roll the sheets or cover back off the front edge while it is up, that way you dont have to completely break the bed down everytime you tilt it up.
Or do like my wife does and make the hubby do it, that way it is never a problem.