MikeP said:
Are the bolts tight because of corrosion? If so soak with de-icer (much cheaper than plus-gas and does the same thing.)
If not corrosion, find the torque settings for the bolts, set a torque-wrench to the value and try that. If that doesn't loosen them, step up the torque value a little at a time.
Heat; Might be best to use some local heat from a hot-air gun if you have one in conjunction with the torque-wrench.
Sorry, but you should never use a torque wrench to loosen bolts (if you don't believe me, read the manual that came with your torque wrench).
When dealing with 'stuck' bolts, soak first. Give them some love taps on the head of the bolt to help the solvent to flow down the threads, wait. Soak, tap, repeat several times. Then try the standard wrench.
If that doesn't work, get an impact driver. If you have a compressor, an air impact rocks. If you don't have air, you can get an impact driver that uses a hammer to function (poor mans impact driver).
If THAT doesn't work, then you add heat. But only add heat to female threads (surrounding area or a nut). Never to the male threads (bolt). Use an eletric heat gun if possible. If you have to use propane torch, be CAREFUL and move the torch constantly and try not to put the flame on the material you are trying to heat up. If the material you are heating starts to glow... you've gone WAY TO FAR.
Once you have heated the female threads, try a standard wrench, if no go, try the impact again.
Good luck. 98% of the time, I've gotten by with just an impact driver.