I have pontificated on RV overloading to the point it has probably upset people. But overweight is overweight, no matter how much a person thinks it "feels capable." Feel free to check my other posts on here. I honestly believe half or more of all TCs on the road are rolling over at least one weight limit. Your family's safety and that of fellow road occupants is too important to play around with overloaded vehicles, and the liability is huge; become involved in a fatality or injury accident, and any attorney worth his bar exam will discover the overweight condition and exploit it, even if it's not the direct cause. Ugly scenarios include a huge judgement against you, your insurance carrier rejecting the claim due to violation of stated limits, and your personal wealth being destroyed. That's exactly what happened to my pool contractor. The 36" tires on his lifted 2500 Duramax looked tough, but had load capacity way below the stock tires. Towing triple axle toy hauler, he had a blow-out.The overload numbers were crazy - nearly 1000 lbs. too much on each rear tire. Only a one ton dually would have been legal on all limits with his monster RV combo, but the factory tires on his 2500 would likely have prevented the accident from occurring. It was tragic. Two people killed, another never to recover fully. He ended up with voided insurance and $3.5M in judgments against him. No more pool company. He had to sell everything just to scratch the surface of paying the injured parties. His wife divorced him, and he died a broken human being, both in the financial sense and the heart.... a lot of guilt and sorrow.
In your particular case, a quick Internet search of 2014 1500 Sierra's (no idea if your's has 4x4 or other options that impact weight) suggested a payload of no more than 1800 lbs., so your 1750 is probably accurate based on your knowledge of actual curb weight and placarded GVWR. Payload, unless qualified otherwise, would need to include your truck's fuel, all passengers, and the gear that you take camping. With four people, that can easily hit 1000 lbs., depending on how light you can pack. Add that to the actual weight of the camper - propane, water, generator if you take one, firewood etc. Using the 1900 lb. estimate for the Vanguard, you're starting overweight before adding any of the afore mentioned weights to your set up. The rig you propose putting together could easilly be 1,200 lbs. or more overweight for a typical road trip. Hello squat and sway! More important, consider axle and tire weight limits. Whatever you end up with, I strongly encourage you to hit the scales at a local truck stop or state facility empty and then fully laden. Guessing does not work. Scales range from free to less than $20 for two axles - money well spent. Your concern regarding the 3000 km journey is good place to start, but you must look at your long term usage plan. My guess is that your 1500 Sierra is inappropriate as a TC hauler, unless you focus on an ultralight pop-up; even then a lot of care would be required to stay in limits. A word about airbags ... they will enhance your handling and seat of the pants feel, and realistically do make a rig safer, whether operating on the fringe of legality or well within the "comfort zone." I highly recommend them for any pick-up or SUV that is going to be hauling significant loads. But don't count them as weight capacity enhancers. They are not. You might consider a travel trailer instead. Or wait until you're ready to spring for a heavier duty truck before getting into a TC. I realize my response is a "buzzkill," but I'd like you to be open-minded to reality. Good luck as you consider your options.