Please don't overside the RCD - there's no reason for this device, if working correctly, to pop a 10A RCD. If you want to follow that solution use a 6" nail instead.

I'm taking you at your word that the fault has been localised to the PSU - get that out of the unit.

Eyeball the feeds to the tube - check no debris or shorts to case.

Eyeball the PSU for debris.

Slave the PSU to a separate mains feed. First with the chassis earthed, then - carefully - without it earthed. That'd confirm any earth leakage through damage to either PSU or PSU enclosure.

Work safely!, 30kV at 30mA can end a life.

EDIT: I once popped a pipe off the tube and pissed water throughout the enclosure. Is there any chance of any residual water in the cooling jacket have been displaced into the enclosure?