I've considred the solar roof, but unless I got the panels and conversion electronics for free, it's not worth it to me--plug-in electricity is cheap.
The trike isn't efficient enough to be fully recharged by solar without a lot of panel, since it's indoors (to protect it from idiots) while I'm at work, and only gets sunlight on it at home in the morning, and then while I'm riding to work for a few minutes near midday. If I'm out on other trips, it'll be parked outside, so it'd get some charge that way, possibly enough to keep it full, depending on conditions, since I tend to ride either in middle of the day, or well after dark.
Takes at least a couple hundred Wh for work commutes; if I had a hundred watts of solar (at the battery charge input point) it could take at least a couple hours to recharge it each commute. I don't know what efficiencies I'd really get, but I expect I'd need 200-300w of panel to get 100w at the battery, on average depending on conditions and time of day.
Anyway, If I already had the panels and electronics, I'd probably do it just cuz.
The trailer's usable surface deck is not any bigger than it was as the MkIV, not yet. I still have to widen it to turn it into the two-dog enclosed cooled MkV. So far the only work done on it was just the stuff absolutely necessary to handle the piano moving. So still only fits one dog at a time; the other still has to go on the trike.
The work to convert it to the full MkV is quite a bit of stuff, and will have to be done in stages to leave it functional in case I need it. Details on that will be over in the MkV thread:
as I get it done, or work out the ideas.