Before you try to drain it out look at your drain pipe. If there is a bend will your bottle go around the bend or will it just wedge in there making a bigger problem?
If the bottle floats I would fill the tank almost all the way full. Then put the toilet side of the RV up on blocks to see if the bottle will float up to the hole. have a few neighborhood kids rock the RV vigorously side to side to agitate the water and move the bottle up.
If the bottle doesn't float I would drain the tank and put the RV side opposite from the toilet on blocks so the bottle can roll to the toilet. If necessary I would crawl under the RV and start smacking the tank with an open palm or maybe a rubber mallet and a small piece of 2x4 to try to bounce the bottle over to the toilet.