Your project is quite ambitious. Sounds like you are in England. In the USA there was a 22 foot motorhome called the Ultravan which weighed about the same as a Nissan Leaf. It got 18mpg with a flat 6 boxer. You could restomod this using a Bolt or Leaf drivetrain including batteries and wiring harness. You would then have an OEM conversion which could be fast charged and have 50 to 150 mile range depending on the donor. The 22 x 8 foot roof would be capable of at least 3kw of solar panels. I don't know if a similar caravan was made in Brittan but it would be a better candidate for conversion. Also if it was aluminium like the Ultravan it would have less corrosion.
I'm trying to size up the conversion of a bus.
I don't yet have said bus but it's likely to be a double decker or single decker if it has to be. Think Leyland Olympian (bus) or Setra s328 (coach).
Average MPG of these jobbies (diesel) is 8MPG
HP for these busses seems to range from 300-500hp and torque around 700nm - 1500nm
Let's say that when it's loaded up I would expect total mass of between 10 and 15 tonnes
Now for my use I would be looking to achieve a range of around 60 miles between charges, used on mostly flat roads but with the capability to make it's way up a mountain once in a while (who wouldn't want to take their bus skiing!)
50mph would be more than sufficient.
So, where would you begin with finding a motor and batteries to carry out such a project?