I seen a Rainbow vacuum at Salvation Army once, I made the mistake of opening it up to see why it had a note on it to leave water in it, oh my god what a smell, I about threw up. I don't understand why people still use vacuums like this that use a bowl of water as a bag. And my plumbing pipes would clog if I dumped that down my drains.
Obviously the rainbow that you came across was not maintained properly. I have a rainbow and a Sirena, if you don't want to dump the water down the drain you can dump it outside. After each use, remove the basin and the separator and wash them in the sink, just like washing a couple of dishes after dinner. Reattach the separator and store the basin separate from the motor unit and the dolly. This will prevent any bad smells from ever having the chance to form. I don't think the oasis has a separator so it would be a huge mess to clean, avoid any water filtration machine that does not have a separator.
Mike