Tornadoes take a lot of research to actually predict and avoid a damage path. They have been a passion of mine since I was a child. I've been studying meteorology for years and the best advice to truckers I have when driving is to find shelter.
Tornadoes are actually very rare to be seen, and very hard to track with an experienced storm chaser group. Most truckers who say they saw one are usually misunderstood, or they are confusing them with smaller "Dust-Nadoes", wall clouds, and straight-line winds from a squall line. Tornadoes have to make contact with the ground to be considered an actual "tornado". If not, funnel clouds often form from a wall cloud or Mesocyclone (all parts of a supercell thunderstorm) are much more common.
If you ever see a tornado in front or behind you, I would look for the closest building available. If you are in farmland, a low laying ditch will keep you safer than your truck. (May sound stupid, but I carry a bicycle helmet with me on the road to avoid getting hit by debris). I've been known to get pretty close to severe weather though. I've seen tornadoes pick up fully loaded semi's with no problem. DO NOT crawl under a highway overpass!
Don't ever stop your truck in the middle of the road. A lot of people do this and cause severe car accidents because most drivers are looking at the sky and not the road.
Depending on the atmosphere, tornadoes usually cause the greatest damage to the north and east of them. This is not always true, but if you are on the south end of a twister, it is usually moving away from you.
Anyway, to answer your question, find any shelter or low laying ditch and don't block the street . They will almost always be safer than your truck. Please let me know if I can be of more help.