Doncaster nudged Moseley closer to the relegation trap door in the Championship on Friday night.
The play-off bound Knights won 38-23 in Birmingham to claim a 14th victory of the season as they look to secure second-spot behind Bristol and home advantage in the second leg of the promotion semi-final, a tie that is likely to be contested with county rivals Yorkshire Carnegie.
Kyle Evans scored the Knights’ first two tries of a tight first half which the visitors edged 21-18.
Mat Clark and Alex Shaw crossed in the second period as Doncaster pulled clear in the second half to earn maximum points.
Moseley need to win one of their last two games to have any chance of staying in the division.