Van based RVs have a box added to the back end of the unibody frame to support the added RV structure. The added part is pretty good stuff, but its still added to the van's boxed sheet metal rear crossmember (most are bolted to the rear crossmember).

You will want to look at the condition of the rear van crossmember (about 2' behind the rear axle) and be sure it is still in good shape, especially if there is already a trailer hitch on the RV. The RV will do OK towing a trailer, but the rear frame is a bit weak to support a large tongue weight trailer.
If your intending on towing a box trailer, or one with a lot of tongue weight, I suggest you invest in the bus style RV.

I'm only a welding shop guy, I've had a lot of van style RVs in because "the trailer hitch is drooping towards the ground more then it was". Sometimes they can be braced, sometimes they can't. A car trailer is at the limit, in my opinion. Gene