One issue not addressed yet is weight. Newer Ford E450s have a max. total weight of 14500 pounds, I'm not sure of the max. weight capacity for the year you are considering. It could be less. The weights of all options and fluids are subtracted from this max. weight to give you the weight of people and stuff you can carry (Occupant and Cargo Carrying Capacity - OCCC), no matter how much room you have. The longer the RV, the addition of slides, and any levelers all subtract from the OCCC. Given a family of four, with growing children, and the stuff you might want to carry on your trips, I would suggest you carefully check out the OCCC of the unit to make sure it can safely carry what you expect it to. A 30 foot RV with 3 slides is going to weigh a lot without people and stuff.
You can also check the date codes on the tires yourself to insure that they are not over five years of age. Other posters have noted this, and I encourage you to follow their advice.
Finally, when you get to check out the interior of the unit, look around the windows and the front over cab area to check for signs of water leaks and damage. This is not a good thing, and can be quite expensive to repair. Best wishes on your RV quest.