Guernsey secured a bonus point at Chichester thanks to a last-minute try from boss Jordan Reynolds as they won 27-10 in London One South.
Blair Campbell, Jason Batiste and Malcolm Barnes also scored tries.
Their 15th successive win means Guernsey stay two points clear of Old Elthamians at the top of the table.
"It was a tough game for us, we had to be in the right frame of the mind and I think we did that from the get-go," player-boss Reynolds told BBC Guernsey.
"Chichester are a very good side, I don't think too many teams have gone down there and picked up a bonus point in the last few years.
"If we'd have lost that we'd have found ourselves in a bit of trouble and we'd be down in second."
Guernsey face bottom side Sutton and Epsom next week while title rival Old Elthamians host third-placed Basingstoke.