[slowmover]

A tonneau is a good start.

I'll assume that a vehicle operator has a handle on annual fuel cost, has reduced cold starts/short trips by combining errands, and has worked out the best routing for commute in 90%+ of driving. Past mechanical issues (steering wander, grabbing brake, etc), there is little left to do which will make a credible gain.

Where aero shows gains to an ANNUAL average mpg is partly to straight ahead and to cross wind effects. Given that highway-only mpg is a significant portion of annual driving. Highway driving that is out and away from a metro area where one lives.

In trailer towing -- as with other things -- it is OEM vehicle design which matters most. Tow vehicle or trailed vehicle. There is no getting around this.

Given that highway travel is in a range from 45-65/mph, that which aids stability has precedence. Muting the effect of crosswinds is highest AFTER combined vehicle brake performance.

The ideal vehicle state at a given rate of travel is in maintaining lane center with little to no steering input. Efforts expended here pay most.

Last is fuel burn reduction where all other issues have been optimized. Too often, I believe, are aerodynamic concerns given weight over the above. As with aftermarket engine tuners, one never solidly confronts the nut behind the wheel. Skewing results of air shaping. The most popular best designed low cost aftermarket Dodge diesel tuner can't really be shown to improve steady state highway mpg. May make the truck more responsive, but the end is that it is no different from tuning the stock carburetor outside factory settings in the 1970s. Gains are to be had, but are never "free".

IMO, treat the combined vehicle rig separately. Unless one is using a trailed vehicle upwards of 30% for annual miles, "simple" changes to the tow vehicle (aerolid, side skirts) and to the trailer (enclosed bottom, skirts) will add margin that noticeably improve wind handling (passing or oncoming vehicle winds the most dangerous).

Getting a "singular vehicle" were it relatively inexpensive would have happened by the late 1970s. Any number of RV manufacturers have gone out of business due to consumer sensitivity to fuel price. There is incentive, but it doesn't make economic sense.

It's the low annual miles towing an RV and near negible effect on annual fuel budget that kills it.

So, if it's good for solo mpg, that's the best approach. If, for the rig, it mediates crosswind problems, that verifies best approach. Yet that is far behind the quality of hitch lash up in terms of results. Safety and FE.

Most RV'ers run few miles. Thus, familiarity, means more also. Being able to predict dynamics in advance. Which is confidence.

I note that thus far are none on any RV forum who casually back under the trailer and take it for a Saturday test run. This is telling. So long as towing remains a special category of activity we'll likely not see acceptable A-B testing (to be generous I'll leave out A-B-A. Anyone serious would incorporate it with a schedule of changes. And devote the time necessary. Not, "well, on our last trip to Lake Goonsquatch, we saw . . . .). This testing is -- solo versus towing -- conducted exactly the same. The benchmark is a 40% drop in FE towing a travel trailer.

Experimenting with light plywood for tonneau length is greatly appealing. Easy A-B testing. Solo and with trailer. May be an optimum length for each situation.
Shortbed and longbed may call for something different. Same for standard versus extended cab. Great room for experimentation.

As a final note, improving in town MPG 20% or more can underwrite vacation towing travel. Start there.
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