[aerohead]

This one is a tough call for me.
All the current research was done within the context of DOT length restrictions.
And what you'll see are forms optimized within this restricted length constraint.
4-feet for rigid,and 5-feet for inflatables.
Fachsenfeld did the most interesting tail I've ever seen,and that was under contract to the Omnibus Company in Germany around 1936.It is an 'extensible' tail.It's depicted in Hucho's book in the commercial vehicle chapter.
It is very close to the 'Template',and the bus measured Cd 0.159 as a model in the Stuttgart tunnel,before Fachsenfeld moved to the FKFS under Kamm's directorship.The Reich Ministry saw his patent and created a shotgun wedding where Fachsenfeld could design as a 'nobody' under the skirt of Professor Kamm's credentials.At one year before the start of WW-II,the German gov't may have been leery about I.G.Farben's ability to provide enough fuel to prosecute a war.They lost WW-I because they ran out of oil.
If I had a motorhome,I'd build a boat-tailed trailer,since there are no length restrictions with trailers per se,and could be left at a campsite area for short excursions until time for the real road trip.
Your existing tail seems very much like GM's 'Optimum tail' investigated by Texas Tech'd Aero Lab and may be performing so well that a very costly redo wouldn't really show at the pump.Diminishing returns as they say.
I'll scratch my head for awhile.We need more case studies!