We have a 2018 Promaster 3500 with the factory towing package. From what I can tell, we can pull a trailer up to 5,100 lbs. We have a 23 ft holiday trailer that weighs about 4,000 lbs. Some of the are...
While I know little about a Promaster 3500, I see a large vehicle, with a 280 hp V6. It should have plenty of payload, but the power train would not be my choice for pulling up, or down mt passes, even with the 6-speed. Maybe someone more familiar with that van will know more.