I have the antenna you linked to - it is mounted on the rear driver's side wall of the trailer using the main part of the mirror bracket. There is a photo of the mount about two thirds of the way down on my trailer modifications
Depending on where you plan to place the Sleek, you will want to insure the attached antenna cable will reach. I had ETI add a 12v receptacle in the face of the cabinet I placed the Sleek, and use a 12V splitter to power both the Sleek & a Millenicom hotspot.
One additional point. When storing the trailer over the winter I managed to break off one of the ground plane radials on the antenna. That, plus wanting to upgrade it to covering 4G frequencies (the original antenna was 3G only) required replacing the antenna. In the 17b, the wiring was run in the back of the upper cabinets & behind the bathroom wall. Replacing it was a difficult job including removing the vent & using an inspection camera to find a path behind the bathroom.
If I was doing it again, I'd request ETI install a run of 3/4" plastic conduit between the cabinets on either side of the bathroom to make replacements easy. With the changes in frequencies cell phone providers are going through, the ability to change antennas is something to consider.
I don't know if the conduit idea is necessary or would work in the 19' or 21', but I'd ask...