I would level the coach for other reasons, but so long as the coach is FLAT (Meaning the wheels are all in the same plane,, (I hope you remember what a plane is) you should be good.
A plane is a flat smooth surface, not required it be level (Can be straight up and down) just if you shine a laser along it at 1" above the plane, no matter how you shine it it will always be 1" above the plane.
The reason for leveling first it to make sure the frame is not twisted.
If the frame is twisted the slide can bind, Frame may twist a bit during leveling by the way, no problem if slide is all the way in.