Keep in mind that 3500 LBS isn't that much. You also need to factor in the weight of the all the gear, passengers, etc. I think even pop ups are close to that 3500 range. I've seen some really nice ones on my local CL and you can even find truck and trailer combos up for sale online as well. A good place that has both the truck and trailer for sale as a combo would be Good Sam website. Many people endup wanting to sell the truck and the trailer as one and sell for good prices.
I guess a few more questions would help. Is your van a 6 cyl? How many are in your family? Once you load it all up and hook it up, your van will feel the weight and strain.
While pop ups are nice and lightweight, they are also more exposed to the elements and they are limited on conveniences like a shower etc plus your toilet is basically in the middle of your kitchen most times.
If there are any RV dealers in your area, go there first and see what they have, new and used and look at weights and ask them what they recommend. You will get a better idea of what you will be able to get by what your pulling vehicle can pull. That's going to be your limitation right there.
Other than that..........RV's are great for the family. We call ours the Memory Maker.
You might also look into Motorhome and then pull the van with you.
Keep in mind there is basic gear you will need and a trailer is like a 2nd home and you will wnat to outfit it with many of the things you have at home. Cleaning fluids, brooms, mop, all of the kitchen utensils, pots, pans, towels, bedding, clothes, and then comes the stuff you will need for the outside, sewer hoses, water hoses, leveling boards or blocks, etc.
There ARE TONS of cool things to add as extras like awning lights, and so forth. Camping World will give you an idea as to just how many things you can buy.
Just in our own we have everything from leveling boards, hoses, vacuum cleaner, all sorts of tie downs, wheel chocks, and the list goes on and on. Keep in mind that all these things add weight. Then there's food. Water in the storage tanks etc. It sounds like a daunting task, but over time you will acquire what you need.
The basic things for outside are the sewer hoses, a hose for city water hookup, the trailer comes with electric cable, some long boards for leveling, wheel chocks, as the very basics.
Also, go to a RV park if you have one close to you and ask someone to show you their rig and how it's all set up. Most people will take the time to walk you thorugh it. If they don't, ask someone else. Hope this helps.