The 1977 Service Manual says under "Main & Auxilliary Waste Tank Removal" :
Note: Check replacement tank for leaks by filling with water before and after installation
- Empty tank completely. If tank is not completely emptied it may be damaged during this removal procedure.
- Remove drain lines
- Unplug liquid level wires (under nightstand or wardrobe)
- From under trailer, remove waste tank access panel
- Support tank with floor jack
- Remove bolts which secure tank pan to chassis
- Pan can be slid forward of the axle to remove tank
- Remove drain outlet from drain tank. It is not necessary to remove slide valve.
- Lower tank by sliding back and down
- Apply adequate sealant between support pan flanges and chassis to ensure a proper seal
- Replace by reversing above procedures
I can't imagine the belly pan supporting the weight of both tanks filled, but I'm not an engineer.
In any event, the "bolts which secure tank pan to chassis
" in number 6 seems to say it doesn't.
In the picture that goes with the instructions, it shows the "pan support main" and a length of U-channel holding up the black tank, and a somewhat flatter pan under the grey tank.
Sounds like a fun job! Good luck!