How did you calculate that, Dave? There should not be much to caclulate concerning the hitch rating and your tow weight limit is your rig's GCWR minus the actual loaded weight of your rig, subject again to the hitch rating limit. If that is what you calulcated, you should be fine.
Horsepower should be on little concern - at worst you will slow down a bit on hills. However, on a 27 foot Brave I doubt if even that will happen. Stopping is the larger concern - once you get that extra 4000# in motion, you will need some help to stop it reliably. That cargo trailer should have good brakes and the MH needs a quality brake controller, something like a Tekonsha Prodigy.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL