I have a 2007 Pilgrim 39' 5th wheel (So there is no manufacturer to refer to nor any warranty to worry about) that comes in at 14.5K GVWR. I normally leave this camper at my site which is about 250 m...
Even with the spring/equalizer suspension, towing nose high can still overload the rear axle. Nose low can overload the front axle. It is best to get the trailer as level as possible and still maintain the required 6" clearance with the bed of the truck.