If you need to trim a foam mattress topper then the best way is with an electric carving knife.
Previously I'd tried with a sharp stanley utility knife (almost impossible to get a decent edge), and with a jigsaw (just chewed it, and didn't get very far), then someone suggested the electric carving knife, and it worked perfectly.
I had previously trimmed a kingsize mattress topper (80x80) to fit the short-King (80x72) in our old pop-up using the stanley knife, but just recently re-used that same topper to fit the full-queen (60x80) in our new 5er, but this time using the carving knife ... so much easier and neater
(BTW using the topper on the "premium" Serta queen mattress ... I really prefer something firmer, but with a memory foam top)
2015 Rockwood Signature UltraLite 8281WS
2016 Ford F-250 Lariat CrewCab, 4x4, 6.2L V8