I have a 1982 Caddillac deville with odd accessories and need some help.

It is Deisel.

#1 It has a RACOR 'fuel water seperator' that says it needs to be checked daily. For what does it need checked? where and how do you check it?

#2 It has an immense gas tank, It has gone down to 1/4 tank and when i refuel it it takes 21-22 gallons (at 1/4 tank) what size tank could it possibly have?

#3 It has an extra 12gal gas tank in the trunk (in addition to the tank mentioned above!) No question about how to work it, just has anyone else ever heard of that?

Well I would take a guess at you having a 30 gallon tank if you have a 1/4 tank left and it still takes 22 gallons or so to fill it up. Unless my math is wrong. Im not sure of answers to questions #1 or #3 but thats my guess for #2 based on the information provided in the post.

Was this car customized to have these parts/accessories you mentioned in #1 and #3 or is that from the factory (never heard of that)?

#2: Diesel Devilles had a 26 gallon tank in '82

I'm not sure whether the Fuel seperator came from the factory or not, I'd say that the 2nd fuel tank is after market though, but nicely installed.