We have an A-class motorhome, the wheel base fits into a standard car parking space, it is 5.7m. The body work may overhang a bit. Being less than 6m is a good size for parking.
Parking at big supermarkets is not a problem, park at the edge out of the way. Public car parks more difficult you may take up 2 spaces but as long as you pay for 2 spaces there is not a problem.
Due to the nature of our holidays (spent in countryside walking the dog) we tend not to park in town centre car parks.
Be aware of car parks that are height restricted, it will not go under.
We thought of trading in our current motorhome for a van conversion, but decided against it as they are rather limited for storage space. We have looked at a number of different ones & struggled as to where we would store elec leads, water containers, boots, coats, bedding etc. Maybe you will be OK with this, but worth some thought.
Could you hire one before you decide to buy?
I do love our motorhome - good luck with finding one that works for you.