Baz
You can do it in one piece down the walls as well if you have good access from the top and the roll of carpet is long enough (depends on how wide the Roll is also). Alternatively you can do the sides separately, starting at the top edge and working down towards the bottom corner of the trailer and the other piece of carpet (floor that you had done), trim with razor as before.
Preferably if you are doing it in one piece start at on one of the sides apply the contact adhesive all over that side and the opposing piece of carpet that would cover that area and then allow the glue to go tacky. Starting at the top edge work your way down to he corner with the floor and add some more glue to about half of the floor area and the same on the carpet, ensuring the glue is right in he corner also. Then push the carpet into the corner, spend plenty of time here, run a big flat head screw driver or steel ruler or similar along the 90 degree bend into he corner to ensure you get a neat sharp edge. Go over the edge a few times working the carpet into he corner and ensure the glue has gone off.
Wait an hour or so then check it and make sure you are still happy before you continue and finish the rest of the floor. Now work from the bottom of the other side to the top edge of the trailer. Trim with the razor and have a beer.
If doing it in 3 pieces I would cut the pieces a couple of inches too big in all directions. If you are doing it in one piece I would leave 3 or 4 inches in all directions so that you have plenty to work with.
Hope this makes sense mate.
Jas
EDIT: I just read hems post and some good advice there. You can also do the floor first as Hem suggested, just make sure you position the carpet in the right position at the bottom when you start. I just find starting at the top edge (assuming its reasonably square) you can be sure and check your overhang there and then be confident it will fit fine.