First off, the weight of the transmission is a non-issue. Also getting the RV up high enough to get the transmission out from underneath is not necessary. I have dropped several transmissions to work on the engine itself and in most cases, all I had to do was move the transmission slightly out of the way to allow me to work on the rear of the engine. The transmission jack that I use, is not a large one, but it is on wheels and has a 1200 pound capacity. This is more than enough for your transmission. Basically all you have to do is put the jack under the transmission and lower it enough to allow you to roll it back a few inches to allow you to replace the rear seal. Once the seal is replaced, you just roll it forward, raise it, and reattach the transmission. Depending on how the engine is supported, you may have to put a bottle jack or two under the engine when the transmission is removed.
