My 2017, 3500, diesel, Auto trans, CC, long bed, 4x4, SRW Ram has a payload sticker of 4365 so no it doesn't have to be a 2wd gas setup to get that.
The tires are rated at 3640lbs.
If I recall my empty weight on the rear is along the lines of 3500lbs I have a tool box (200lbs), a headache rack (150lbs) and includes my b&w hitch (150lbs), and a b&w hidden gooseneck mount (75lbs). The toolbox and rack some of that weight is on the front axle since they are directly behind the cab.
Real question is which 5vr does he have as they have 5 or 6 different lines some are much lighter than others. If the pin weight is going to be 3200lbs as previously suggested could certainly be close or over. If he has taken into account the added pin weight and added to the brochure spec already then he would be fine.
However you didnt ask about those questions and regardless of all that I have no idea why the other brands would have so much less payload, 3400 does seem light to me. As far as tow ability I doubt you will see much difference while towing. Hp is HP, torque is torque, braking is braking and in those regards they are pretty similar. Unloaded you may find the higher payload truck rides a bit stiffer and maybe not. As you put it logically it would stand to reason that it would fare better if it wasn't working at max capacity but seems logic falls out the window many times. I am not sure I would expect any real difference in capabilities beyond the payload factor.
2016 Jayco 28.5 RLTS