We tow a 08 Keystone Springdale 6200 lbs dry weight with a 04 suburban 5.3 liter and 3.73 axel. Its at its limit in the wind or up steep grades. Turning
the air off helps a lot. On smooth flat land its no problem at all even with
the air on but my next vehicle will be with a larger motor. We go to Branson
often and climb those hills pretty well with the air off but it gets very warm
down there and can stress everyone out. I'm surprised at how well the 5.3
handles the load without the air on so its a question of how comfortable you
want to be on the road. The new expedition 2018 looks like a great vehicle
for towing but I would talk with others before buying one to see how well that
new turbo six handles your RV. They say the HP is higher than the old expedition
and the torque as well but we will wait to see if folks agree. I talked with
a camper who went through several transmissions on new Ford truck, they were
replaced under warranty but he couldn't use his RV all summer. That's another
thought. Ask around this could just be a one time experience and he could have been over loaded which puts a lot of stress on everything.