Is a dual rear tire F350 or equivalent the only way to go? Are all/most 3/4 ton trucks overloaded and potentially top heavy with a 2500+ pound camper attached?
Your dealer is correct. Its a fed "recommendation" anyway and has no legal ramifications if the owner chooses not to follow the recommendation or the rather silly Camper load rating number which is low even in a F350 DRW. I doubt if many are under the TC payload number with any truck.
Our 250/2500 trucks have 6000 to 6500 RAWR for carrying weight.
Ford gives their F250 a 6100 RAWR which includes the axle/wheel/tires and rear springs. If the F250 has the heavy service package its the same suspension as a F350 SRW truck.
Most are good for around 2800-3200 lb payload before exceeding one of those ratings.
Actual numbers depends on the trucks actual axle weights. A regular cab 2wd long bed will give a bit more payload than a crew cab long bed 4x4.
Ask around for your brand truck campers wet weight numbers as I'm sure these guys have one just like it