First, sit in the trailer at the dealer and imagine a full day there. Is there enough room for everyone to not be up each other's noses? Is there enough storage? When the kids go to bed, can you still use the trailer - have a drink, read a book, maybe watch TV without waking them up? Sounds silly, but some bunkhouses have doors that close and some have curtains - big difference.
Are there AC ducts in every room? One TT I had didn't have one in the bathroom and it got very warm in there.
Make sure that all the connections, valves, etc are easily accessible when the trailer is fully deployed (slides out, etc). One of mine had the tank valve handles UNDER the slides.. Little things like that get old VERY quickly.
A backup camera is a very wise investment - not just for backing up, but for ensuring that lane changes are safely made.
A ladder to get on the roof is very handy.
No matter how much storage you have, space will be at a premium. Anything that saves space is worth considering, from the number of tools you carry (and you WILL need tools!) to the length of hoses and how many kids scooters and toys you bring.
Get an Electrical Management System (I recommend Progressive Industries) and a water pressure regulator. The EMS is pricey, but less pricey than a new AC if the power fluctuates and kills it. The water pressure regulator is cheap.
When towing, go slower than normal and leave lots of room in front of you. Towing a trailer requires extra vigilance because there much less margin for error.
We have a rule for the kids - when camping, no electronics except for right before bed and right before breakfast, or when it's raining. They go outside and PLAY! We also keep a lot of movies on board for rainy days and late evenings when we are camping with friends. The kids watch a movie before bedtime at one trailer while the adults socialize outside.
Splurge on the items that make camping easier - whatever that means to you. For me, an electric awning is SOOO much nicer than a manual awning. I have a bit for my drill that fits the stablizer jacks, making them easy to lower and raise versus the lug-wrench hour-long affair. The ice machine eliminates the need to (have to go) buy ice, and it's faster and easier that doing it in the freezer. Andersen Levelers make leveling easy. And if I didn't have an electric tongue jack, my first tow would likely have been my last.
The argument over whether or not an SUV is capable of towing a trailer rages on... But from my experience, it's less about towing capability and more about towing SAFETY, and wheelbase (in my case) matters. The difference between towing with my Jeep Commander and my Silverado 2500 is literally night and day. With the Jeep, the drive scared me to death and had me stressed and aggravated on arrival. With the Silverado, I barely know the TT is back there. I arrive stress-free and ready to enjoy myself versus looking up the nearest therapist in the Yellow Pages.
Lastly:
Read these forums - a lot - you will learn a metric ton of knowledge here that you likely will NOT gain through experience - certainly not the painless kind, anyway. This forum has saved my bacon numerous times and given me great ideas for improvements to the trailer and just better ways of doing things.
