One other advantage of NEW
A) it's new, no one elses DNA in the mattresses, etc...
B) you can finance new!
I'm against taking on big debt especially when you have children who need a college fund, but if you have some money set aside to pay for a newer used trailer, you may find that you can get a brand spanking new one for a big down payment and very reasonable monthly charges!
Either way, GO FOR IT.
I went Airstreaming with my Aunt Jean for only a few weeks as a child, and those weeks still stand out in my memory. Get the kids involved - stabilizer jacks are the kids job, so is setting up the outside camping chairs. I watched a child who might have been 3 years old "help" daddy set up the trailer - and the look of pride and joy on her face was too touching for words. Everything is an adventure at that age.... and you'll have more fun learning to see things through your children's eyes.
By all means get a book of birds and another of butterflies and moths - "collecting" where you saw your first of every species, or even finding what to look for on your next trip can be lots of fun, and teaches the kids some basic geography, botany, etc.