I added a class 4 to my Gtown to carry 2 scoots and a total tongue weight of 600lbs. +
At the time I did a lot of research including with Ford and THEY were ok with 600 lbs+ provided I upgraded to a class 4 welded on AND my particular coach did not use welded on fram extensions of any length that would be weak points.
I ended up getting my coach inspected by a good hitch outfit that does nothing but hitch work...and then I got a 2 cycle/scoooter carrier from Joehauler.com due to the width issues I had with the scooters. the Versahauler tracks were no wide enough for mine...you might want to check the measurements center to center with the bikes side by side to ensure enough clearance...but Versahauler makes an excellent product.
The class 4 hitch is designed for 1000 lbs tongue weight...but the coach is the limiting factor. Get your frame checked for those extensions by a competent guy and then no worries...I've noted NO driving or sag issues with mine. The hitch removal of the class3 and install of the class4 completely welded ran about $350 so I thought that was reasonable. Pix here:
Two Scoots...one G'town