Here is The York (Off-Street Parking Places) Order 2013 (https://tro.trafficpenaltytribunal.gov.uk/TRO/York/YC143.pdf), this car park permits:
Camper Van not exceeding 3050 kilograms gross weight and not exceeding 2.0m in height
Is your vehicle within these limits?
This is a very simple case, draft reps:
Dear City of York Council,
I challenge liability for PCN YC9356319A on the ground that the alleged contravention did not occur, owing to inadequate signage. When I parked my vehicle I used the app to pay, and I did not see any signage indicating restrictions on motorhomes.
I note that the signs dotted around the car park which carry the location code, such as this one
(https://i.imgur.com/j5KOBCm.png)
do not carry any information about restrictions on motorhomes. When I paid on the app, no warning message or other indication was given that my vehicle was not permitted.
Therefore owing to inadequate signage, the contravention did not occur.
Yours faithfully,
Pop this into a PDF and upload it to the council website as an attachment, don't forget to get a screenshot of the confirmation screen.