As one that upgraded from a tent trailer to travel trailer to 5th wheel, I think you should look at 5th wheels and a big truck. Right now we have 3 kids in the back of ours and they do fine. They tow much nicer and wind does not seem to affect them like a travel trailer.
Have you thought about renting a truck and going for a road trip to see how you all fit for a longer drive?
If you are full time, you will want the separate bunk space for the kids to have their own space. Its nice at night to be able to close the door and still be able to use the living space without waking them up.
You will also be amazed at how you will seem to fill the space in a larger trailer with more stuff and more weight.
One thing to think of is where you plan to go. Some spots are hard to get into with longer trailers if you get too big.
