Has it worked before?
Several recent threads here where it appears the fan isn't moving enough air to pick up the sail switch, which is necessary for the gas valve to open and the igniter to light it off.
Usual problem is either a crushed heater vent hose or camping at high altitudes.
You don't say what model TT you have. Some folks have the heater under the stove, with a cold air intake that consists of slots in a wooden panel front. Some folks have reported that the removing the air intake cover will be enough to get it to light off.
If there's restricted air flow, improving it at either the intake (cold) or discharge (hot ) side can be enough to pick up the sail switch.
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