Last year we purchased a 2010 Cambria 28B that is 29' 7" long, 10' high and 7' 11" wide. The bed is 60" x 75" and is on a slideout. You can see the Cambria floor plans for various years here:
Itasca Motorhomes - Brochure Archive
We find this size works well for the two of us. It will not get the sort of fuel mileage your now getting. About the best we see on the interstates, driving 60+ and pulling a 2100# vehicle is 9 to 10 mpg.
2010 30' Itasca Cambria 28B, Blue Ox track bar, Hellwig sway bars (front & rear), Raybestoes/Spicer Dual Axis adjustable bushings set for +8-1/4 deg. castor (ds) & +8-1/8 deg. castor (ps). Air bags, ScanGauge II.