You should stick with a regular utility van or truck with a canopy. Ambulances have high gross vehicle weight because of that rear section with all the equipment and people. That doesn't translate to towing capacity at all if you still have that rear cab on there.
Wait, can you explain this? I don't know anything about trucks, but I figured they were built to lug all that heavy furniture (cabinets and benches) and equipment around. Without all of that, doesn't that put you in a better place?
It's my understanding that the gross weight of a vehicle is the maximum weight, including the cargo capacity, not the curb weight of the vehicle. Therefore, a vehicle with a high gross weight would be perfect, as long as you weren't loaded up with cargo.
You are correct in that the gross weight is the maximum weight of the vehicle and that also includes the tow capacity.
My concern for the ambulance chassis is all that frame rigidity and support is meant for a cab or compartment where all the extra weight is. If that cab is still on there, even with most of the gear removed it's going to be very heavy and take up most of the GVWR (gross vehicle weight rating). If he were to take the rear cab off and run it just an open frame chassis he could net decent towing capacity but there are two problems. One, the chassis didn't come outfitted for towing so it probably lacks a weight distributing hitch set up. Fairly easy to weld one or bolt some custom stuff up yourself but still it's a concern that the vehicle wasn't meant for it.
Secondly, he could run into traction issues. That rear axle is a dually, set up for a burley amount of weight and with out any weight on it (a 500lb tongue load from a trailer isn't shit compared to the 5-6000lb rear cab) will put hardly any load on the suspension and sacrifice traction. I don't know what the term is for it, it's out there. Thats why you see people put sand bags in the back of their dually if they're going to launch a boat or something. They need that weight to aid in traction to keep that power on the ground.