Tossel registered a brace before half time, and completed his hat trick just two minutes after the break to put the visitors into a commanding 37-9 lead, but the Valke fought back bravely to register four unanswered tries in the second half.
The Leopards, however, were able to ride out the storm to seal a bonus-point win that sees them climb to third in the North Section standings, while the winless Valke remain second from bottom.
Things began on the wrong foot for the home side, as Tossel needed just two minutes to get his first, and flyhalf Gerhard Nortier made no mistake with the conversion.
Nortier added an additional penalty to the score before Tossel bagged his second in the ninth minute to take it out to 17-0.
Adi Jacobs, playing on the wing, then rolled back the years as he darted over the whitewash three minutes later, and the hapless home side were now facing a daunting 24-point deficit.
Fullback Angus Cleophas was able to add a trio of penalties to give the home side something to cheer about before the break, but Nortier himself added a further three pointer of his own to put his team in command at 30-9 by the time the players broke for half time.
Tossel then needed just two minutes to complete his hat trick and the bonus point in the second half, but it marked a significant shift in momentum, as the Leopards appeared to take their foot off the throttle.
Grant Janke dotted down for the home side in the 53rd minute, and although Nortier added a further penalty soon after, Sandile Ngcobo scored for the Valke to keep them in the contest.
Anrich Richter was next to cross the line in the 76th minute, but, crucially, Cleophas had failed to convert any of his team's three tries, affording the Leopards some considerable breathing space.
It meant Drew van Wyk's bonus point try in the dying seconds counted for little more than consolation.
Scorers:
Valke: Tries: Janke, Ngcobo, Richter, Van Wyk. Penalties: A Cleophas (3)
Leopards: Tries: G Tossel (3), AJ Jacobs; Conversions: GJ Nortier (4) ; Penalties: GJ Nortier (4).