The loin section of the pig makes up a majority of the pig's back, and is separated into three sections: the blade end, the sirloin end and the center portion. The loin is the most tender and delicious section of the pig and is susceptible to being overcooked and dried out easily. Luckily, you can slow roast a pork loin by smoking it over indirect heat. While this technique is normally used on charcoal grills, you can modify your propane grill to accomplish the same results.
Prepare the pork loin prior to preparing the grill. You can marinade the pork for a few hours before grilling or you can just cover it in pork rub. How you flavor the pork won't affect the smoking process.
Remove the grates and warming rack from the grill. Place the small loaf-shaped cake pan all the way to the left over the burners and fill in the rest of the space with the lasagna trays. You should be able to fit two lasagna trays into the grill.
Place one of the grilling grates back over the lasagna trays longways, leaving the cake pan easily accessible as this is where the wood chips will be kept and they will need to be replenished during the smoking process.
Place the wood chips in a bowl and allow them to soak for about 30 minutes. Put a handful of soaked wood chips into the bottom of the cake pan. Allow a little room between chips to ensure they aren't smothered so that they smoke properly.
Cover the cake pan with aluminum foil and poke holes all over the top. Place the cake pan back in the grill and fill the lasagna trays nearly to the brim with water or another flavoring liquid.
Set the left burner on "High" and wait until smoke begins to come out of the smoker. Once there is smoke, place the pork loin on the grill over the lasagna trays and close the lid. Place the electric thermometer inside the grill and adjust the burner to acquire an inside temperature of 225 F.
Remove the cake pan and replace the wood chips every 45 minutes or so. Allow the meat to cook for about three hours and then turn it over. Continue smoking the meat until the internal temperature reaches 160 F on the meat thermometer.
Remove the pork loin from the grill and allow it to cool for about 10 to 15 minutes before serving.