It's been quite a while, but I figured I'd update this for the benefit of others asking similar questions.
Following this thread, I ended up pulling the trigger on a solar system. I have a 100W panel permanently mounted on the roof and a second folding 100W that we can deploy if we really need to stretch it. All this is connected through a SunSaver MPPT controller to our battery bank.
Most of the time, the rooftop 100W alone is enough to keep us at 100% or very close. If it's very hot (i.e. the compressor fridge is working overtime) and we're looking to stay out for a week or more, I'll bust out the 2nd 100W and that does the trick, especially if I adjust it towards the sun a few times a day.
So now I don't really care about truck-to-camper charging. Problem solved!
Ethergore, I still don't think the CTEK boosts the voltage. That said, you asked "what's the point if it doesn't boost?" and I believe it will at least bring the batteries back more quickly from very low SoC vs the VSR. This is due to the fact that the CTEK should stay on deliver a maximum current (assuming proper bulk absorption behavior) regardless of the SoC, while the VSR will cycle when the SoC is very low. Even if it doesn't boost past the alternator voltage, that should still get you very close to 100% during the bulk absorption phase with the added advantage of ramping up the voltage slowly in order to keep you camper batts healthy.
As for your observation that under load, the voltage increases the closer you get to the source battery, that's totally expected. It's basically DC line loss. The further you get from the source battery, the more resistance you have due to longer wire, etc. Per Ohms law, that higher resistance translates to a bigger voltage drop.
My issue is that the voltage drop seems quite high (0.7V), which is likely due to the small-ish wiring used by FWC (12g, I believe). If I were really concerned about maximizing the efficiency of charging off the truck, it would probably make sense to redo that with larger wire.
And yeah, I understand that you can do parallel solar panels to keep the voltage under the CTEK max. That said, a real controller (such as my SunSaver) gives you the flexibility to do either parallel or series, which is nice. Again, it doesn't sound like the CTEK is a very good solar controller.