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

I’m in agreement w the others here, if that camper is 3 tons that’s a bit much for planned, prolonged use.
However, when shopping, I’d be totally comfortable up to around 5klbs with the right add ons. The actual axle rating is likely around 10,000lbs and the remainder of the chassis and drivetrain is up to it, save for tires and wheels.

That truck leaves you many options for larger TCs with slideouts. Just not the heaviest ones on the market. Congrats on the new rig and happy shopping!