If I get a chance I will post a picture of my 1983 T-1550 shower pan bottom. I believe you may have the same pan; 23"x23" +/- a fraction.
The bottom is black ABS thermal formed over a series of pegs (loosely speaking) giving a honey comb like effect. This in turn is glued or bonded to the cream colored ABS floor pans bottom side.
The black base sits on the floor, I assume over most of the 23"x23" area with a rough opening for the drain trap/elbow or however it was plumbed. Not just sure what that area looks like.
The PO took the shower floor (pan) out and replaced it with plywood and vinyl tile finish floor, thus I have the pan albeit damaged in its removal.
To answer your question as well as I can. If your shower pan doesn't have any cracks in it and it isn't flexing unduly in any areas as you step on it you are probably ok. Look very carefully for minor cracking. The creaking could be from some glue joints popping loose or a shift in the sub floor allowing movement.
TT:1983 Sunline T-1550
TT:1996 Sunline T-2053
TV:2005 Toyota Tundra Double Cab 2UZ-FE i-Force 4.7 L DOHC (MFI) V8 4WD SR5 Automatic
P3 break control
"I know a lot about nothing and nothing about a lot"