What's the outside temperature where your rig is at? If its cold enough, the 20# bottle isn't going to be able to put out enough propane to maintain pressure. heck, a 30# probably would work either.
You see, the colder the tank is, the harder it is for the propane to vaporize thus why you can light but not maintain a flame. A larger tank will have a larger diameter this more surface area and consequently more available gas vapor.
2000 Cherokee 29BH with 6V batteries, LED lights & 400 watts of solar power, flipped axles and raised. 2007 Tundra 5.7L DC-LB with lots of mods. C-co, 8/158th AVN Maint.