Hey, Brian, 3rd time's a charm! I love my 1988 17-Bigfooter.
To your question: my 1988 has a spec sheet showing the Fresh Water tank is 28 gals. (but standard is 23; this has the optional bigger tank); Grey Water 17 gals.; Black Water 8.5 gals. Also, the water heater holds 6.2 gals.
I should think the year difference in age probably doesn't make a difference?? Hope this helps.
BTW, if you'd like a scanned copy of this Spec Sheet, I'd be happy to email it to you. Pretty much what it lists is the "amenities" for the 3 models available that year; Dimensions (length, headroom, tire size, stuff like that); then Tank Capacities, which I just gave you.
Good luck with your new Bigfoot!