We have a 3/4 ton gasser and stopped fretting about mpg long ago since we don't get anywhere near the mpg your experiencing.  Even when gas was nearly $4 a gallon several years back we didn't think twice and took our kids on dozens of RVing trips annually, we fully understand fuel costs can be pricey, but the memories these excursions provided are simply priceless.  
Our tow vehicle is dedicated to hauling our 33' 9K# TT and sits in the garage otherwise, just got her out as weather is finally warming up and we are going out for our first trip of the season this weekend.  Kids are all grown, working and/or going to college rarely having time to come along.  My DW and I still love RVing, now our trips are geared for our interests and passions.  
Never calculated our mpg while towing or otherwise, and with minimal time off from work to get anywhere when RVing with the kids, driving slower to gain mpg was something we didn't bother considering.  Not that it would matter much anyway to any RVing plans we have this year, after having our 3/4 ton gasser serviced for our trip, went to a Chevron and gassed up for 2$ a gallon.