2003 lance 1171 and 2017 Srw f350 am I asking for trouble?

I think you are asking for disaster.
1171 is the same size what 1161 I carry and the "wet weight" in my case is over 6000lb. To add to the trouble, the COG comes behind rear axle even I observe the heavy items loaded up front.
Meaning you can expect adding in 6500-7000lb to the rear axle.
Now go and check what are your ratings.