Tire pressures for long gravel roads?

[07EXPYEL]

My scout leader dutys will have me on about 150 miles of back roads this summer. To avoid flat tires and excessive damage I plan on airing down my tires (Michelin LTX 255/70R18) to about 25psi, perhaps as low as 20psi. I lost one pricey $$ Michelin last year on a gravel road running at highway pressures. I just wanted to get feedback on what is a good safe pressure to run at. I will just have passengers, and towing a very light utility tailer with <300# tounge weight. I'll fill back up to normal pressures before I get on the highway.