Maybe I am a bit cynical, but I can see a lot of hard standing pitches being reduced in size to become motorhome only pitches as often we do not use the sun canopy or awning, so effectively there will be more pitches per site.
Jersey is small you could save ferry costs by reducing the length and leaving the car at home, just moving the caravan to the site on the mover But seriously a lot of the roads are very narrow like Cornish farm tracks be and there is no google street view so you can reccy the roads before you go so pick your site carefully.
It will be fairly irrelevant on a global scale what ever we drive here, because for every single person who gives up an internal combustion engine in Europe it will be replaced by 2 more in the emerging economies ie India and China and these economies do not have the infrastructure to support anything other then internal combustion engines for the domestic market, it will be like the 50's here I'm sure some of you remember when your family aspired to owning their first car. I'm afraid it will not change much until oil runs out.
The parking issue will depend to a degree on the length and width of the motorhome, ours is roughly the same size as a supermarket delivery van or ambulance so apart from avoiding height barriers not an issue.
We changed to a Motorhome last year after many years of caravanning, and I don't think we would ever go back, the most important thing is to realize that the way you holiday will change we now pick sites within cycling (electric bikes) distance of attractions and tend not to stay on sites as long about 4 or 5 days instead of a week before we move on. Before we purchased we looked into what we needed something not to long under 6 metres fixed bed, proper shower, large kitchen worktop, large fridge 150 litres, the ability to seat 6 7 people round the table, 550 kilo payload seems like a fairly impossible list! but we have it in our Benimar Mileo 202 which has an electric bed that drops down from above the lounge to seat level ready made at bed time. Turning up on site with whole van heated from the drive in winter or cool from the aircon in summer, 130 litres of drinking water already in the tank, just driving onto the pitch hand brake on and ready to relax is a pleasure faster and easier to drive on the motorway and the fuel consumption is better then it was for us towing.
We always disconnected the van electrics from the car and put the van handbrake on as it lessens the swaying if its a bit choppy, If the vans brakes are out of adjustment the ATC operating rod can drop and jam the brakes on.
According to the government web site https://www.gov.uk/government/news/improving-air-quality-in-cities it does not affect private vehicles. Mind you I live in Southampton and have not been into town for 6 years the parking is atrocious
There are quite a few models out there so you might not need to go to a tag axle this is one model that with the right plating will give you over a ton of payload so you should be able to carry the mobility scooter http://www.marquisleisure.co.uk/motorhomes/stock-item/benimar-mileo-294
We changed to motorhome from a caravan last year, and in the UK you still pretty much have to use proper campsites although there is a growing number of pubs and restaurants allow you to stay over night if you are eating. However cross the channel and it a completely different story, nearly every town and village has some sort of facility's for what they call camping cars. Would we change back to a caravan its very unlikely, the ease of setting up and departing, and traveling far outweighs the disadvantage of not having a car, we just find somewhere to stop near to what we want to visit and either walk there or ride the electric bike we carry on the back.