if your vaporizer/converter is freezing up that is caused by lack of coolant in the system. The coolant helps keep the LP gas warm enough to prevent the freezing.

As for the engine cutting out, the freezing converter will cause that.
As for the coolant leaking and no external sign of the leaking?
Check the exhaust for any sign of excess moisture, steam or water coming out.
Check the engine oil, color should be normal golden color or darker if it has many hours on it, if the color has a milky or almond color you may have a head gasket issue allowing the coolant into the engine oil.

As for the pressure in the radiator? if the engine is hot there will be pressure there even if the coolant is low.
One way to check the cooling system for leaks is to use a cooling system pressure tester. Make sure the system has coolant in it, install the pressure tester on the radiator and pump it up to the recommended pressure for that system and then check for leaks.