I thought about them but didn't bother after researching.   
 Air tabs have been tried. People I met  that have used them didn't notice a mpg advantage but a cleaner rear of the RV . They also might help a little with stability in the wind but IMO, it would depend on the TT they were installed on. 
As far as measuring MPG, the road, weather conditions would need to be identical to measure accurately using specific equipment other than someone's fuel gauge and how many gallons they had to pump back into the tank. Take those opinions with a grain of salt.   
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/23208771.cfmhttp://www.rv.net/forum/index.cfm/fuseaction/thread/tid/23094344.cfm