I think you're good on payload. Even with the short wheelbase your van should be stable. I would highly recommend upgrading to Bilstein shocks. Then OEM shocks are garbage, and you'll really feel that with a heavier trailer.
I think you'll be really close on combined weight. The dry weight doesn't include "options" such as:
Customer Value Package (Mandatory)
13500 BTU Central Air, 30 lb. LP Bottle w/ Cover (2), 6 Gal. Gas/Electric DSI Water Heater (10 Gallon in 38BHDS & 38FDDS), 8 cu. ft. Refrigerator (Residential Fridge in 38BHDS & 38FDDS), Bath Skylight, Bathroom Vent, Bedspread, Cable/Satellite TV Hookup, Digital TV Antenna, Microwave, Outside Shower, Oven, Power Tongue Jack, Spare Tire, Tub Surround, Water Heater Bypass
I really think you'll probably end up closer to the trailer's GVWR than you expect.
Power wise you'll be ok. But the Express is cooling limited on both the engine and transmission. The Class C owners have discovered Hayden makes a better fan clutch that keeps the engine cool. GM has a recall out for the transmission which reduces torque converter lockup so it won't burn up the clutch, but that will probably make it run hotter overall.