I get about 18 highway if I keep it under 70 when not towing. Every year we do a 2000 mile round trip to Florida pulling my 6500 lbs camper. I average about 9.5 to 10 mpgs pulling right around 68 mph. Keep in mind the east coast is pretty flat.
The Tundra has plenty of power. Gas tank is actually 26 gallons, the low fuel light comes on with 6 gallons left. For me. I find that stopping every 3 hours for fuel isn't that bad. I have a 2 year old daughter and a pregnant wife... they rarely last past 3 hours without needing some sort of break anyways.
If you are able to keep your payload numbers low enough, the Tundra is great and I would recommend it to anyone.
If your payload exceeds your ratings... its up to you to decide if you want to go over the weight ratings and stay in a Tundra or move up to a bigger truck. Honestly, outside of the Ecoboost or max tow GM, all half ton trucks have roughly the same payload. I wouldn't lose sleep if I was 150 or 200 lbs over my rating. My axles are rated for a little over 1000 lbs higher than my GVWR, and my E rated tires are rated for 5,000 lbs higher than my GVWR.
If you don't want to try the Ecoboost, you will need to step up to a 3/4 ton or higher for additional payload. Then you are talking 2500 to 3500 lbs of payload, as opposed to 1500 to 1800 lbs of payload in a 1/2 ton.
The trade off is a heavier truck will get worse gas mileage empty than a lighter truck with same drivetrain. The heavier truck will ride heavier (although I hear the Ram with coil suspension rides pretty nice). Registration fees will be higher, etc.
Do whatever you feel comfortable doing. In Florida I always used to see a couple guys from up north (Michigan and Indiana, I think) that towed decent size 5'ers with their Tundra's. I know they were well over the yellow sticker payload rating. They would drive 3000 miles round trip every year and never had a single problem with the Tundra. I don't doubt it can be done, I just don't want to take that liability upon myself.
If I had to replace the Tundra right now, I would get a Ram 2500. Softer ride, large back seat of the mega cab, 5.7 or 6.4 Hemi engines produce plenty of power and they offer double the payload for when the kiddo's get older.