Hi, seems to be plenty MHs stay at Seaham CU 1 overnight which is closest to Seaham Hall itself.
The beach there is still very popular for collecting sea glass so before covid, you would often see a few foreign plated vehicles staying.
There are no restrictions , as far as I am aware ,on the other Seaham car parks but there does not seem to be many overnight there, probably because of the proximity to the town itself. Victor