Just playing with the van idea, I went on to Ford site and priced out a brand new, 2013, E350 with V-10. Added lots of options including tow package for Class IV Hitch, integrated brake controller, etc. Even brand new list with options is under $40k, so agree, a 2 or 3 year old van could be quite reasonable. And with the large family, no one is crowded.
FYI - the V10 is the same motor I had in my Fleetwood Tioga that weighed in at nearly 17,000#. We pulled a motorcycle trailer with two Harleys loaded from Texas to South Dakota and back and average 8.5 mpg. My guess is that the mileage will be comparable to the Suburban.
One more thing - don't beat yourself up on the inexperience. I was an "experienced" camper and purchased a TH based on advertising and salesman hype, and ultimately ended up having to purchase a new Tow Vehicle to be happy/safe. It happens!
Happy camping!