Probable best just to install a 15k unit, best in addition to the one you have, if there is another roof vent opening. The Maximum you can expect is at most a 20 degree drop between the intake and the outlet. In the area you are talking about, unless you are in full shade, one 13.5 unit will not handle the load. Either get another unit, get a bigger unit, or another cold beer. These units just don't have the air handling capacity for a 30' rig. Probably OK in the shade, but in full sun, no way. Sorry, but that;s the way it is.. Notice in my sig picture, there are two 13.5 units on top, with 4" at least of roof insulation.
