The fuel mileage is going to be hurt some. The square soft sided block storage will give you the worst mileage. Tapered or molded fiberglass or aluminum will be less hurt to mileage, Any fabric exposed to constant moisture will be more susceptible to mold or mildew. Fabric will have UV damage. I have had 5 different RTT's Autohomes; Overcamp; (all Fabric), Columbus; Fiberglas top and bottom Fabric sides,3 different Extreme; Fiberglas top and bottom Fabric sides. I have borrowed other fabric tents to try out.
RTT's are not the same as a ground set-up family tent. They are for sleeping only. Yes, I have sat in our tents and played cards with the wife, dog and myself in the RTT's. If you want the room of a ground tent, get a awning with a room for under it.
RTT's have their place and there are OK- RTT's, good - RTT's and great - RTT's. It all depends on what you want to do. If it is quick set-up there are some out their. I have been timed with many people timing me. Autohome - Columbus. Close up me sitting in the drivers seat time starts when the drivers door opens. I walk to the back flip the rear latch open, use my thumb to push up on the top shell. The struts finish the rest of the opening.Walk to the passenger side of the vehicle. Reach up grabbing the later hook the top hangers on the edge of the door rail. Drop the lower section of the ladder to the ground. Climb inside where the sleeping bag is waiting for me and I am inside the sleeping bag. Average pace this takes me about 25 seconds, door to in sleeping bag. If i rush I have done it is as fast as 19 seconds. This is the Autohome, Columbus model. I am sure that you have looked at the tents on the market. Most of the have a 5 minute setup and no room for much inside storage in them. The packing the Tents away range fro 2 minutes to 12 minutes and some really you need two people to pack it away.
If you go with a fabric tent with a fabric cover I would not recommend that being left on 365 days a year. I would suggest that you byu a 10' X 20' shelter to protect the tent cover.
RTT's are not for everyone! I have 22 years of using the Autohome brand, I am very happy with them. For my working years I averaged over 250 nights a year sleeping in my tents. My tents we only removed from the roof of my vehicles to be placed to the trailer hard top.
Since retiring I average maybe 80 nights a year sleeping in My RTT. At 76 years young I am slowing down a bit.
Before you buy a RTT see if you can find a friend to borrow one and actually try it out.
A warning the all soft tents have wind noise the better soft ones have less noise then the cheaper ones.
There are also disadvantages to any RTT. The center of gravity is higher. Do not expect to drive like you would a sports car. I know this a s for a number of years I had sports cars. Fuel mileage can be bad if you drive over 65 MPH. My JKU, I would get 24 MPG staying at 62 MPH holding the gas peddle steady. The Jeep has 4" lift, 33" tires winch, Gobi rack, Jeep winch bumpers front and rear. If you drive with a light foot and not use the cruse in the mountains you can get great mileage with the tent on the roof.
Total mileage of the tents on the roof of my vehicles are over 650,000 miles that is on pavement and off pavement.
Disclaimer; Autohome was one of my sponsors when I was working. I have been retired for a while now and I have not found any RTT any better then the Autohome tents. You get what you pay for, buy cheap get cheap.
If you have any additional questions I will be happy to answer them.