looking for Champion/Firan Telstar specs and tank help

Got my hands on a 95 Telstar that needs work. 28 ft class C. I bought it because I knew the superior construction they used back then with the one piece cabover shell with no seams to leak, as well as the sprayed foam insulation in the walls. This particular unit has barn doors in the back and a garage area. Never seen a Telstar like that and was wondering if it is custom or if they were ever made like that. Also, it has no black tank, previous owner put a wet bath in and stubbed the toilet but left me with nowhere that I can see to mount any kind of a black tank underneath. The roof is metal, and I see no vent stack at all. There is a grey tank, but I have no idea the capacity of it and it's covered in spray foam. There is a vent pipe coming off the side of it, but it terminates with a short pipe directly under ther rv which i think is odd and probably not correct. With the driveshaft in the way most of the length of the rv, I'm looking for ideas on how to mount a black tank. Right now there is a 45 degree pipe stub coming off the toilet which isn't the greatest as it should be a direct line under the toilet. I'm thinking I may have to shave the spray foam off the grey tank and possibly stub the toilet into it to combine black and grey. Nowadays it seems most holding tanks are shallow but long and wide and there's no way that will work under a class c. Any thoughts?