General rule to all trailers: longer it is, easier it is to control when reversing. Shorter ones turn much faster but result you knifing your trailer tongue and rear tires in no time.
To go straight: periodically turn your steering wheel left and right little amount, this should keep trailer going straight (turn about 45 degrees both sides is my gut feel)
Like others said with turning tongue axle your trailer behaves opposite from single axle trailer. To reverse these 2-3 axle trailers in mudrunner is challenging because trailers tend to follow track paths automatically (like they would in real life). The key to success is patience and this tip: when your tongue starts to be max 30 degrees angled from your truck you need to "CHASE" the tongue meaning you turn your truck wheels opposite direction and follow trailer's path. When tongue goes straight trailer goes straight but if your truck is not inline with trailer the trailer will soon start to turn opposite direction where you just turned.
Practice is key but it takes usually about whole week in real life by self training in field to figure these out. You may have theory down in your head but in the end it is gut feel and experience that let you manipulate trailer successfully.