All Dodge Mega Cab/Quad cab and regular cab trucks have the same 6000 RAWR.
The 2500 Mega cab can weigh 2900-3000 lb on the rear axle which leaves 3000-3100 lb for a payload.
I would load the family and make a trip to the scales and get the trucks separate front and rear axles weights. That way you know what your dealing with.
A single gross weight is useless unless your state has a gross weight registration.
Oh yeah....trucks don't carry weight determined by the truck makers GVWR or a GVWR payload. Carrying weight is a function of the separate FAWR or the RAWR on the trucks certification placard.
All Mega Cabs have the same 6' 4" beds as do the quad cab and regular cab 2500 short bed trucks.
The 2500 Mega Cab is a popular truck for towing any trailer including heavy 5th wheel trailers (over 10K) and heavy truck campers.