You unlock Monster Trucks two ways, 1) Placing first in all of an events series and 2) reaching the point targets during a championship season.

To unlock them all, you have to aquire 1080000 points. It may seem like a lot, but you should be able to obtain this easily by going back and replaying your low scoring rounds. A couple points of interest about the point system. You get the points regardless of what position you finish in, but you only get NEW points. So if you score 40k on one event and then replay it and get 42k, you will only receive 2k more points towards your total. Once you get 1080000 you will unlock the last truck (assuming that you have already beat the world finals).

I found that replaying the Eliminator races on the Southern Series helped me the most. The midwest series also has a lot of obstacles for you to run into, especially trees and jumps...

speaking of jumps, when you are trying to string together combos and you dont have anything directly in front of you, I've found that running into a wall at a diagonal will launch in the air, and give you the air time trick...just a tip.

Fairly simple, you need to hit the large obstacles in the races that give you the "Monster Slam" points. Examples are: Water Towers, Large Vertical Storage Tanks, Air Traffic Control Tower, Large Chinook Choppers, B52 bombers, Pirate Ships, Schooners, The engines of a 747, Support Beams for Buildings (some), Large Grain Conveyors....Just get 8 of them and this will pop up. You should get this by the second series if not before.

Same as "Outta My Way!" but you need to get all 20.

I want to thank NeonInVermont for his tip in THIS thread. The large bridge after the ramp (its at the end of the pipe during the South East Finals) was my last one, and apparently a lot of others. To ensure you get this, you need to make sure you are in the lead and slow down a bit when you approach the ramp, otherwise, you fly right over it. Once i hit this, the ach unlocked.

**I highly recommend using Grave Digger to combine the Grave Digger Fanatic Achievement**

For all the competitors and contenders, you need to finish all the events, after the 8th one, you get the achievement. If you are finishing first in all the races, you will also get the champion achievement at the same time. There is no specific order in which you need to complete the competitions (other than only a few are immediately available, and other pop up after you completed the previous ones), just do all 8 and the achievement pops up.

Level Specific Tips:

There are a lot of shortcuts to be found on these races, keep looking for ramps/billboards/water towers that you can take off to the side, they really do help. This is the first series, and is pretty easy in comparison to the later series, so take your time and learn the little tricks and handling features of this game.

***See South East Series Competitor***

Level Specific Tips:

These maps are chop full of trees, jumps and shortcuts. This is an ideal series to really pump out some points. Remember that if there are trees there, you can go there. Be careful because there are some drops, but follow billboards and side jumps to hit these shortcuts. I rarely used the actual path for these races and it worked out well for me.

***See South East Series Competitor***

Level Specific Tips:

I found this series to be a giant pain. You really need to hone your cornering skills in this one. I have found that when you are turning, to hit the x button and then follow it right up with boost and it will shoot you in the direction you are facing without drifting. Look out for the Pirate Ship, Vertical Silo Containers, and the schooner for the Monster Slam obstacles

***See South East Series Competitor***

Level Specific Tips:

This is another level where you get some big points. It takes place in an outdoor hangar, and has a subtle area 51 appeal. There are cliffs but they are not as dangerous as the water on the coastel series. Lots of jumps and shortcuts will help you dominate this series. Be on the lookout for these monster obstacles: Stealth Jet, 747 (engines), space shuttle, Alien ship and chinook helicopter.

Same as the South East Series Competitor but needs to be on Pro mode.  To switch to pro mode, from the main menu select options->difficulty->Pro.

I highly recommend starting your championship series as Grave Digger. You will get this once you complete every race with Grave Digger as your truck. If you have already started the series, all you need to do is hit Y to switch the truck at the start of the race screen and choose Grave Digger, and go back and complete all races that you played without GD as your truck. After the finish of the last truck, during the points tally, this will pop up.

Same as Car Crusher. I received this after I finished the Championship, these numbers do not carry over when you change the difficulty. I highly recommend going back to the World Series Final and just grinding it out;if I had to guess, there are anywhere from 50 to 80 cars to be crushed on the last one. Just keep on playing the freestyles and this will eventually pop up.

 ** If you have to go back for the Unlock all Trucks achievement, this should pop up during that depending on how many levels you need to replay for points **

Same as the Costal Series Champion but needs to be on Pro mode.  To switch to pro mode, from the main menu select options->difficulty->Pro.

Once you switch to pro mode, you dont have to change it again until you restart your xbox360. 

You get this event once you complete all the Series. It has tons of obstacles, but a very small area to deal with them in. It took me to about this one to realize that score doesnt mean everything. First run I got a 110k points in one combo and still got a 36. My advice is to link up and get at least two combos over 15k. Remember to do all the tricks, and hit all the obstacles. Be careful because it is very easy to get out of control and head into the stands, which will make you lose your multiplier.

Another tip: When you are in the middle of an obstacle that is a smash but on level ground, try to do a donut in the middle (turn one way and hold ). During the donut, you can hit other objects and it increasing your point total and your multiplier.