As best as Irecall, hitch weight when towing a trailer should NOT exceed 10% of tow weight. IE: 2000lb tow limit = 200lb hitch wt limit.
And as someone else pointed out, if you're hanging all the weight above & behind the hitch, the allowed weight is cut in half !
An adapter plate is sold on EBay that fill in the roof ridges for the gen2 TC. The plate makes a flat service to mount the fan. Mounting the fan on the plate raises the fan around 3/8". The plate plus a shim should let the fan close. Ford Transit Connect Roof Vent Adapter for first generation 2002-2013