There are two "closet bolts" holding the toilet down to the tank cover. one is in front, just behind the foot pedals. that's the easy one. the other one is in back. You might be able to access it through the access plug in the rim of the toilet. Open the toilet seat and lid, and you'll see a "plug" like thingy toward the rear of the toilet bowl on the left hand side. If you remove that plug, you can look down in there and see the other bolt. you "should" be able to get a socket on a long extension on the nut that is holding it on, but I couldn't on mine, because the bolt was so tall, there was no way to get a socket over it, as it stuck up very close to the underside of the toilet bowl. So I had to contort myself to get a box-end wrench on it from behind. Then I found that someone in the past actually cut an access hole in the skirt on the left hand side....you don't see it, as its so close to the wall there. even through that hole, it was still tough to reach. anyway....that's how you do it on a 1973 safari land-yacht. not sure what your trailer is...never heard of a "land cruiser" before.
can't help you with the rest; what you're describing would take some extensive modification in my trailer, as the vanity, which is connected to the shower, sits on top of the black tank/tank cover. you'd have to re-design the whole bathroom, really. good luck!