A little extra weight has practically zero effect when the roads are flat. The only extra fuel cost is from an infinitesimal increase in the rolling resistance of the tires. Climbing hills, though, makes a bit more difference since that extra weight is actually carried to the top (and you have to brake it going down again). Even so, on a modest 30,000 lb coach + toad, we are talking a 2.5% increase in weight. You probably still would not notice it.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL