If it is 3500 dry I bet you will be at or above 4,000 all loaded up and ready to travel. That weight will depend on if it included options. Also will depend where the fresh water tank is and how much weight it transfers to the front axle.
I'm in the same situation. I'm right at payload and under axle caps, GVW, etc. But I am so close I wish I had gotten a dually. I added a BigWig but honestly haven't used the camper enough to be in any situations that I was uncomfortable. Maybe 3-4,000 miles of use so far.
It will also depend on what brand truck you want. Go look at trucks on dealer lots and make note of capacities. I don't want to start a truck war, but I found Ford had lowest, GM next, and Ram highest payload caps.