Could be the thermocouple is bad. When the furnace lights, the flame heats the thermocouple, which expands the gas in it, and applies pressure to the gas valve, which keeps the gas flowing. If it doesn't apply the pressure in 30 sec or so, the gas valve closes (at least I think that's how it works). So there are 2 issues I know of: 1) If the flame isn't heating up the thermocouple, the gas valve shuts down. Check to see that the flame is engulfing the thermocouple. 2) If your MH or heater is somewhat older, the flame may have burned through it. This seems less likely, as the thermocouple in my old home furnace used to last several years with a pilot light continuously heating it. I highly doubt your MH furnace has that much use on it.
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