Airing down to 20 lbs. is not even airing down. To obtain the benefits of airing down, you really need to be lower than 15 psi and for a 33x12.50 tire, you can safely go down to 12 psi as Jerryj above recommends. Just avoid driving on the highway afterwards any higher than 25 mph for any distance as that will cause the tire to get hot and possibly fail. It'd be best if you can air up before leaving the area so I'd carry a decent air compressor with me.
Caution that inexpensive (cheap) air compressors will take a LONG time to air up your tires, upwards of 40-45 minutes is pretty typical with the typical cheap air compressor that brags about the high PSI it can pump to but says nothing about the volume of air it can pump. In reinflating tires, air volume is king but that it can pump to 200 or more psi is useless/misleading information. A pump with at least a couple CFM (cubic feet/minute) into 30 psi is highly desireable. Some pumps brag they can pump lots of CFM into 0 psi (basically a flat tire) but quickly fall off once the pressure starts to built in the tire.
But any compressor is better than no compressor.