I think an EV trailer would be of little use if it can extend the range only some 60 miles.
A Tesla S can do 200 miles by itself. Why build a trailer with less than that capacity?
If the pack would be that light you don't need a trailer at all and just have it in the car (the other option for EV mild range extension).
By the time you've done over 200 miles you'd really need to stop and rest somewhere. Only the bluntest of car journalists would not take the opportunity to fill the packs.
Sure, there are not that many quick charging points yet. You may have to plan ahead or make do with slow charging 110V (even though you can charge both packs at the same time). It may be slightly cumbersome, but not prohibitive. Not for us hypermilers anyway
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2011 Honda Insight + HID, LEDs, tiny PV panel, extra brake pad return springs, neutral wheel alignment, 44/42 PSI (air), PHEV light (inop), tightened wheel nut.
lifetime FE over 0.2 Gigameter or 0.13 Megamile.
For confirmation go to people just like you.
For education go to people unlike yourself.