Last week we bought a 2" memory foam topper. We put it on and didn't use the mattress heater. Temps went down to about mid-40's. With flannel sheets, one blanket and a comforter, we were toasty. Not sure if the mattress heater would have "gotten through" the foam topper if we'd used it, but it certainly wouldn't have hurt.
Last year had just the mattress w/ heater and upper-30's and were fine.
In both cases, we had an electric heater turned on with the temperature setting down low, and the furnace set at 64 degrees as backup. It ran a few times.
If I were you, I'd hook up the mattress heater and if you need it, turn it on. I think you'll be OK.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10