In just the two years since then, prices on batteries and panels have dropped 25% or more, and solar power per square foot at a good price point has gone up significantly (400W monocrystalline panels can be gotten for $200, in the same form factor as the 200W panels I had been budgeting for). I've now lowered my budget to $4000 for the same setup I was planning to spend $6000 on two years ago, and with 400W panels, I no longer need to upgrade to a larger RV to begin the project.
This summer is almost over, so I'm going to wait until spring to start assembling my system in earnest. Anecdotally, this is a game-changer for me. I'm looking toward year-round full-timing starting next summer, because I can now afford the power I need and don't need a larger RV as soon as I thought I would.
I intend to buy undeveloped land far from civilization in the next few years, and I'm now confident that I can DIY a whole-house solar and battery setup so cheaply that access to mains power won't be a factor in deciding where I settle. Even with seasonal variation in power production, I'll manage just fine, and the system will pay for itself in well under five years. In fact it'll pay for itself instantly if you discount the five-figure cost I would otherwise have had to pay for running a new mains power line far into the woods. And by the time I pick some land to settle on, I'll already have enough solar on my RV that I won't even need to augment the system initially; I'll be able to power a small house in a temperate climate directly off the RV itself, while I build a larger solar array (likely ground-mounted to avoid regulations and insurance complications related to roof-mounted setups).
I know my situation is unusual, but the fact that any of this is possible for well under $10k is a huge change from even a decade ago.