Overnight filled to the brim with tap water somewhere if it does leak/burst won't damage anything like out in a driveway or yard with the underside properly supported for weight distribution & relatively level. Since you should be leak testing it somewhere other than it's final destination in case the worst happens there's no reason to use RO/DI or salt water because it will need to be drained once you've confirmed it is in fact not leaking and ready for use (just grab a hose).
Ongoing monitoring is always a good idea: Running Neptune LDK
sensors and a separate IoT leak sensor system from Amazon around the base of our tanks/mixing station. If they ever do decide to leak I want to know about it ASAP.