I ran across this online and sent it to AeroStealth for his consideration,with respect to his upcoming RV 5th-wheel boat tail project
As you can see,by tidying up the underside of the trailer,a cleaner onset flow is delivered to the tail region,upping the overall drag reduction.
AeroStealth's RV is already belly-panned and has lower sides than a conventional Trailer van,so he's got a shot at a better drag reduction than a semi.