A lot of thought-provoking discussion!! To start with Steve's comment, our trailer is so small that there is very little room on the roof for permanent panels. That fact, plus the "very shady campsite" factor, means that for now we will stay with a portable panel.
The big take-away from the earlier posts is that I have some more homework to do. I will have to test the cables with a semi-depleted battery, instead of a fully charged battery. And I will have to test not only the voltage but also the amperage. (I wrote myself a "non-expert's cheat-sheet" on how to use my multi-meter as an ammeter -- if anyone is interested in that cheat sheet, just say so and I will post it.)
I am surprised to hear that 13.5 volts won't even charge a semi-depleted battery -- we have found, after much real-life experience, that my 40 foot cable will do a fine job of charging a battery at around 13.9 volts.
I know this for a fact because we test our battery (at rest) with a voltmeter almost every evening, after the sun goes down. It's almost always at 12.7 volts. Even on a bad day (some clouds or partial shade), it's at 12.5. By contrast, in the morning, it tests out at about 12.3 or so.
The obvious solution to all of this is to bring both cables but then to deploy the 70 footer only if needed -- otherwise, the 40 footer will be preferable. The problem, though, is that the 70 footer is very bulky and heavy, and storage room is at a premium. So if it causes a significant voltage drop, and will not do a very good job of recharging the battery, then I'm better off without it.
