For those who might be interested, here's a quick description of the project from start to finish.
The original plan was simple enough: replace the original "three-way" motorhome special fridge (no longer functioning reliably) with a regular domestic "mains electric" fridge. First, find the right fridge. It had to fit in the space available (obviously
) but also match the existing interior design, and come at a sensible price. I got lucky, very lucky, with the upgraded version of a Hisense model I'd picked out last year, which was identical in size, to within half a centimetre in every dimension, and came in a brushed steel finish that matches the cooker, the cabinet handles and various other trimmings.So I've gone from that:
to this:
The empty space contained two particularly useful features at the back - an existing 230V supply, coming from an electric hook-up via the main control panel; and a vent in the floor
Not at all coincidentally, that vent has exactly the same diameter as the hole into which a rather useless extractor fan over the cooker fits, so the extractor fan found itself re-located.
It's purpose there is to help keep the fridge working properly during hot weather. It'll suck cooler outside air from underneath the vehicle (usually parked on grass at those times of the year) and force it across the compressor. That's still a bit of a work in progress, as the fan is about as feeble in this role as it was when supposed to be venting cooking vapours
but it'll do for now.The next challenge was to provide the "off grid" power - easy enough with a massive 200Ah LiFePO4 battery and a 2000W/4000peak DC-AC converter. As long as they didn't take up all the floor space. The solution was to build a "secret compartment" at the back of a press under the main wardrobe, that's never been of much use because it's too low and too deep. Now it has a worthy purpose in life: I installed a partition in the lower unit, cut the floor of the wardrobe in two, installed a tray in the hidden recess and strapped the battery down, out of sight and (mostly) out of mind.
The inverter (inverted!
) is fixed to the face of the partition, keeping the cable length between itself and the battery to just 50cm to minimise voltage drop. 
Then came the hard part: reconfiguring the electrical installation in such as way as to provide "real" mains power to the fridge when a hook-up is available, aswell as charging the battery; to provide "off-grid" mains power when not hooked up; and (preferably) to not have to think about switching between the two. And then, because why do things the easy way if you can make things more complicated, I decided that it would be even more useful if I could also use my electric kettle and microwave while way out the back of beyond, or up some lonely mountain, or even just parked outside a supermarket.
Cue much neck and brain ache trying to decrypt the factory installed wiring scheme to identify which cable went where, all of their junction blocks being at the limits of my reach inside and at the top of the wardrobe, but in the end, I was able to separate and remove four circuits from the original mains supply and re-route them to a shiny new consumer unit, tucked away at the back of the wardrobe, along with the solar panel's charge controller and a thermostat to manage the fridge ventilation. All the trunking, sockets, cables and junction boxes came from the "might come in useful" stash (couldn't believe my luck when I found 6mm² black and red stranded copper cable hidden inside some heavy duty "twin-and-earth" I've had for about twenty five years
)
It's sooooo pretty when it's all lit up
... which is one reason it's all hidden away. AliExpress provided me with an automatic transfer switch (ATS) at a good price, and a surge protector/real-time load display; the local hardware provided the empty box and three trip switches. There's one for the fridge, one for the microwave, and one that protects both the (single) kitchen socket and an outdoor socket, which I've never used ... until this week. There's also a spare socket on the inverter from where I can draw power without going through the consumer unit.
The ATS is wired to assume that a mains hook-up is the default state, so it'll always switch "back" to that if one becomes available; but as soon as I unplug from the mains, it'll take 230V from the inverter which can be left permanently switched on while I'm away from home.
Out of sight behind the fridge (but accessible through the big side vent) is an additional socket that is only connected to the "real" mains, into which is plugged the battery's own charger. This is the one point that still needs to be addressed, as the cheapy charge controller supplied with the panels doesn't kick in until the LiFePO4 voltage drops to about 30%, and I no longer have any way to charge the batteries from the vehicle's alternator. There are expensive black boxes available to deal with both of these situations, but I'm going to see how the system copes under different real-world conditions before I commit to one or the other.
In the meantime, all that was left was to securely fit the source of all power: the panels on the roof. Now four, because I managed to damage the first two that I bought. Their performance is significantly degraded (maybe a little better since I attempted repairs last week) but I've installed them behind the two new ones to add rigidity to the support frame, though they're not actually connected up as I don't have the necessary adapators.
In good order, two panels give me a confirmed output of 12A in bright light, and they're not too fussy about orientation; on cloudy days, less or a lot less. The fridge draws about 0.5A for about 20 minutes out of every hour, so theoretically even the two damaged panels should be able to supply enough to run it during the day. In practice, with no top-up, the battery alone can keep the fridge running for four days in hot/very warm weather, which I think is "satisfactory" although it'd be less so if I found myself under gloomy grey skies for a week and without the possibility of charging via the alternator.
Then again, on my last long trip, I got a 100km more than usual out of a full tank of diesel - have I spent the last seventeen years spending a fortune on a diesel-powered fridge?
) around the appliance.