[hillbillyhotel]

hello david
lets see if I can help, when you pull the slide valve on the thetford , only about 1 to 1/12 gal goes into the pipe, so the rest is still in thetford tank. so no, it does not empty all the way, but like others, if it is full you can pull valve and buy a little more room,
I have never had a splash or slosh out will traveling to dump station with valve open, just close the lid for extra safety , but I guess if it was full enough and road rough enough, anything can happen though, just never heard of it happing before.
hope this helps.