Here are some pics
of a '73 Sovereign
Your's ought to be the same.
There are two main frame rails, 5" rolled C-channel approximately 60-61" apart, connected by 4" tall crossmembers every 24". There are 14" long outriggers jutting out from the frame rails to support the walls of the coach.
The main frame rails are welded to the A-frame and run the entire length of the coach to the bumper.
You can see in the picture below where the fresh water tank sits just forward of the axles, supported by 2" angle and a 1" plywood platform. The black tank will be found between the main frame rails just forward of the trunk. (Assuming a rear bath)