But, for starters, it sounds as if the old tent is kaput. Right now you're just blowing money on sealants and the like that aren't working. Seems to me that your tent's original factory-installed waterproof layer is gone.
Both tents you mention, the Eureka Getaway 9 ($190 at Wal-Mart) and the Coleman Sasquatch ($248, currently on sale for $186; www.coleman.com) are good family tents, sleeping up to eight or nine people. The Coleman uses a coated nylon for the floor, a very reliable setup that should keep water out in all but the most torrential of downpours. The Eureka uses polyethylene, which is inherently waterproof. You shouldn't need to do any additional sealing or anything elsejust get them in the car and go!
As for the air mattresses, it's actually surprisingly difficult to create a completely airtight membrane out of lightweight materials. Take tubes on bicyclesalmost all of them lose a little air each week. Still, an air mattress certainly ought to hold enough air to keep you comfortable during the night. You probably have some pinhole leaks somewhere. To find, fill a bathtub (a kids' wading pool would be better), inflate the mattress, and start holding sections of it underwater. Sooner or later you'll see the telltale stream of air bubbles. You should be able to find a mattress-patching kit in the outdoor department of any large store to help you plug the hole.