I would look for one with the 5.7 and the tow package. Even if you are looking for used, there should be some out there. I do believe that you won't suffer in the mileage department either. My brother tows with an older Tundra with the 4.6 and I get better mileage towing with my 2008 5.7.
With the tow package and weight distributing hitch I tow my 7500 GVW trailer without a problem.
2008 Toyota Tundra, Double Cab, 5.7L V8
2006 Airstream 25 FB SE
While you are probably underestimating weights, the 4.6L should be fine. However, I highly recommend the 5.7L as idf22 said. Fuel economy difference will be negligible. Power increase will be significant.
A & A parents of DD 2005, DS1 2007, DS2 2009 2011 Suburban 2500 6.0L 3.73 pulling 2011 Heartland North Trail 28BRS 2012 VW Passat TDI