Get one where the walls have piping and a pump to push the centrally heated fluids through those. I have spent several days in winter with ice and snow outside (12h a day in the toy expo in Nuremberg, Germany, the rest of the time in the RV) en it was actually very comfortable inside the RV. It used a RV-sized small gas tank and we refilled it once a week. Shower, cooking plate and fridge could be powered by either gas or electricity in that RV, which could handle a max of 6 people sleeping (3x2person bed) in it
Solar energy did cause a problem though. During the day it kept the battery charged, but at night the voltage level dropped, tripping the car alarm, which wouldn't shut off at all. The next day we were rather urgently asked to leave the camping spot and pay a fine for disturbing the rest of the other people staying the night there. Better find out if you have that kind of problem with your RV before you embark on your 12 month journey.
You have different types of solar panels. While most still generate power with daylight alone, some panels are generating more than others. There are even panels that can generate a small amount of power when it rains on them. Those are much more useful in the UK, I would assume.

Depending on the size of the RV/motorhome you will reduce your energy consumption. Mainly because there is much less room to even have big appliances. Or computers. Laptops won't be that big of a deal, energy-wise or space-wise.
Good insulation and electric devices that only use the infra-red spectrum (instead of the common radiating units) help a lot with keeping the RV warm in winter too. These spend much less energy to keep you comfortable, are much more durable than the standard electric heater and you won't wake up with a major headache afterwards. Consider those if you have an RV without plumbing to heat you in winter.
If you must use a fuel powered generator, get an external one and forgo the use of the engine in the RV. External ones are build for the task and are much easier/cheaper to replace if something does go wrong. And make sure you ventilate, day and night if possible.
Last, but not least...keep your spirits up. Living in rather cramped space requires a certain mindset, which a lot of people think they have or can work on. Two/three weeks everyone can handle, but longer periods, such as 12 months in your case, that is much, much harder. Do not underestimate this, the money you'll be saving by living this way, could easily be spent on divorce settlements afterwards.