Having the camper level may work fine for some people with slide outs and water coming in, but I read many years ago in a owners manual from a previous TT that you should put a slight angle on the slide out so that any water will run away from the TT slide out opening. Will also help keep water drained off from condensation accumulation overnight. Been doing this for years and works very well. This does not mean that you can not check seals or caulking, preventative maintenance is a must.
Good luck and Happy Camping