I have a 78 31'
Sovereign with a rear bath that I just went through this. The pan is actually the support for the tanks, not just an access panel. You should search around for a service manual, (as opposed to the owner's manual) for your model to get detailed instructions. It's a job, especially without the right environment. With that being said, I did it myself in the side yard of a friend where I keep the trailer. I had leaks at the dump valve for both of the tanks, and ultimately cut the rear plumbing out to aid in the removal of all the crap. I knew that I was going to have to replace the valves anyway.
Once I removed the pan the tanks dropped down toward the front of the trailer, only supported by the pipes connected at the back. I had already removed the toilet and the toilet flange that screws down into the black tank. There is nothing more than the rear pipes on the grey tank. In the end, I used the scissor jack out of my truck to hold the front of the tank level until I could get the rear pipes to disconnect from the tank. Then I simply pushed the jack over and let the light tank fall to the ground.
More than you asked for, but if your 29 is like my 31, understand that the pan is also the support, and not just an access.
Mike