Everyone has different needs. I'm not a fan of TT that have the dinette on the left side. All you are going to see out of the window is your neighbors campsite. I'm also not a big fan of slide-outs. To me they just seem like more hassle to set up and more stuff to break.
I like a simple basic box with the dinette on the right side , the bathroom in the back, the fridge and stove on the left side, and the bed/beds up front.
My 20 foot TT has these qualities and weighs 4580 pounds loaded. I've had it on a scale. I usually get 9-10 MPG towing. I'm just a weekend camper that stays close to home. After getting to the park, driving around seeing the sites, and towing back home I average about 12 MPG.
Careful driving will gain you more MPG then the shape of the trailer.