welcome...I have a 73, as well. the switch by the door is a bit "un-intuitive". its not quite a 3-way switch. what it does is turn power on to the light fixture that is over the sink. when its in the off position, that light won't work at all, no matter what you do with the switch. when its "on", you can turn the galley light on and off with its own switch. I think the idea is that if you want, you can leave the galley light "on", and activate it from the doorway....that is "code" for house-building. when you're entering the trailer in the dark, you don't have to fumble around for one of those other overhead lights. just flip the switch at the door.
black tank: not sure if you can actually fit anything bigger in the allotted space, but let me know if you can. So far, we've found the 13 gallon tank to be big enough for our weekend outings. I've done quite a bit of investigating on the grey tank issue...so far, the hardest part seems to be finding a pre-fab tank that will actually fit, and still be of useful size. You can have one custom made, of course, but its REALLY expensive. I got a quickie-quote for >$400, and I'm not sure its really worth the expense. But there IS space in there...it is a "do-able" project, I'm sure. search around on the forum for lots of threads on the subject.
dropping the belly pan is not too difficult...more of a PITA, than anything. I had mine down earlier this year to "investigate", and what I did was drill out the rivets up to the x-member just aft of the rear axle, and then just rolled the sheet metal gently toward the front of the trailer. when I was done, I just rolled it back, and re-fastened it.
Now, what I found when I was under there was that the rear edge of the plywood floor had pretty much disintegrated from past leaks.
you can't really see this area from inside the trailer, due to the layout of the bath. I've done some temporary repairs, but at some point, I will need to remove the bathroom entirely to do a permanent repair. If your tank is leaking, you may have the same problem. I'm still not sure of the source of my leak. It may have been multiple different leaks that have come and gone over time...but the plywood was still wet when I found it last spring. It doesn't appear to be the tank itself, though. I just had it filled w/ water and black-tank enzymes, to give it a thorough year-end cleaning/flushing. water level in the tank stayed the same for 2 weeks.