I just have a Group 24 marine battery on my TT. We don't dry camp much at all, so it provides all I need for emergency break-away service, and a backup if I ever needed to run the fridge on gas without being hooked up to the TV, etc.
It has saved me a couple of times, though. DS loves to listen to DVDs while playing around the TV at the campground. On a couple of occasions, I have run the battery down in the TV (before I bought a higher capacity one). I would pull the TT battery out, set it on the ground in front of the TV, hook up the jumper cables, and in less than 10 minutes total, I was back in business.
There are several reasons to have a decent battery in your TT. There is no reason not to, unless your TT is so small it doesn't have brakes.
