Not true, quite a few big name RU sides ground share, London Irish and Saracens do for starters.
Hudds Town share with Hudds Giants. Wigan Athletic share with Wigan Warriors. Hull City with Hull FC. IIRC Everton reserves play in Widnes.
There's now also Widnes Vikings FC, played there first North West Counties Division 1 game on Saturday, which is the very bottom of the non-league pryamid, not sure what the long term plans are or how closely the clubs are linked.