The Traveler is a self contained system...the toilet sits atop the tank, so there's no way to bypass the tank. So he'd have to replace it with a toilet, the PuraSan and a small (10-15 gal) tank to use if/when he visits a "no discharge" area or marina...and all the related plumbing.
Raritan does offer a "bundle" that combines the PS with a tank...the Hold 'n' Treat system.
Raritan Hold N'Treat Bundle I'm not crazy about it because everything flushes into the tank which can be pumped out or emptied through the PuraSan (or ElectroScan). IMO that defeats the whole reason for having the ability to treat and discharges each flush.