Our first TT had no sofa. Our first TT lasted exactly one season... :(
We moved up to a longer TT to get a sofa and also to get a slide for more space. It was a very good move... :)
A sofa is one of the most important things in our TT and gets constant use (two adults).
Note that the typical "jacknife" sofas in TTs are NOT very comfortable and aren't very well built. We replaced ours with a double recliner with power recline.
Also note, that unless you kids are different, most of them prefer using a tent next to an RV. A sofa might only get used by the parents.