I opted to purchase a 1994 Suzuki Sidekick 4x4 to flat tow behind my Sprinter. Only about 2,200 lbs. as compared with 4,000 lbs for my 2005 Jeep Wrangler 2-door Unlimited. I love my Jeep, but didn't want an RV big enough to easily tow it around. The cost of buying the Sidekick was well under the increased cost of buying a bigger RV.The tongue wight would be minimal, I would be flat towing a Jeep. Except for when braking but even the the brake controller in the Jeep should handle too much force on the tongue. In the end the goal is to be as safe and within limits, and it does not seem they can do it.
Harry, I have to correct a few of your specs.A coach house platinum III might be among the highest for tow ratings but it is still under your 15000 GCWR goal.
It is built on a Ford 3500 cut away van body and uses monocoque construction for rigidity.
There are some small mid size class C builders that will up size the platform to a 4500 if you ask but that still might not be enough.
I haven't seen anything built on the chevy silverado 4500 / 5500 series yet but that platform is probably the strongest out there short of the class A type RVs. Those are being built with a surprisingly wide range of frame lengths and WR since it is literally 2 big beams front to back..
You are right - I was confusing the Platinum / IIICoach House Platinum III is on a Ford Transit chassis and does have a very limited towing capacity.
Platinum II is on a Sprinter
The Platinum can be had on a Ford E350 or E 450 chassis.
They are pricey but nice coaches
They will even build the E=series out as four wheel drive