Our 78 Ambassador 28 has the water filter installed as an option for the original owner, and it's an Everpure unit with a replaceable cartridge.
It looks something like this:
Everpure Water You Can Trust
There's also a listing for an RV water filter on their site, and it looks like the H54 unit.
The filter heads haven't changed, and the current Everpure cart's will fit.
I just bought one for our rig at an RV store for about $32...which was cheaper that the same cart. at an appliance store I checked on.
Goggle the Everpure and you will probably find one near you at a decent price...
Ray in California