With cargo weight of 1500 lbs. in the van and a tongue weight of 600 lbs, pulling a 5000 lb trailer you are going to be struggling just a bit on hills with that 350 engine. I'd also suggest you keep the transmission in "D" when towing. At least to start. When you get more acquainted with the loads and performance, you can be shifting into "OD", especially on the flat when on Interstates.
If you don't have one, a transmission temperature gauge can alert you to inordinate transmission temperatures before you burn up another transmission. I'd suggest to get one installed and make sure you have a big enough transmission cooler. You are operating right at the maximum for the drivetrain of that van.
