First, I don't understand the dealer's "connection" between the furnace and the water heater. I don't see how the furnace would ruin the water heater. I would assume you would actually be using them both, because the water in the water heater is going to cool off faster than usual.
I'd say set the infrared for one temp and the furnace for a lower temp.
The mattress heaters work great. Just don't expect to slide your hand under the covers and have it be hot like an electric blanket. It's only about 150 watts (same as a large light bulb). But it will keep you warm. We haven't been to 29, but we've been in the upper 30's.
It will also prevent condensation under the mattress.
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