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...
I think you'll be fine on main highways. Secondary roads might not work. At all. Pulling that much combined weight with the front wheels is asking a lot when roads are wet. I've seen Promasters not towing, get stuck on mild inclines in wet weather.