i have a bad coil pack as i unplugged cylinder 2 and the idle didn't seem to change now my question is do you have a part # for the coil pack so i can order one thanks in advance

another thig you might want to check before you rebuild the l.p. system You said the plugs showed alot of wear. while the truck is running unplug each coil pack 1 at a time and see what happens.
sounds kind of like a bad coil pack. each pack unplugged should make a difference in the running till you get to the bad one. if you have a bad one, once its unplugged there will be no difference in the idle.

There are few variations of the vaporizers and most of the components are the same to rebuild them. If you have done the Impco ones then you should not have a problem. Just make sure you have all of the correct components. Nissan offers a complete kit but it will only work on some of the original versions. One of the biggest things is making sure that the pressure settings are correct. I believe that if you can get a copy of the service manual section for the vaporizer it will give you step by step process and the correct pressure settings.
As far as the grade of fuel a lot of the LP suppliers use what they call farm grade which would be similar to what is used with your LP grill and is not very clean so it has a tendancy to build up the sludge or tar. The reason they go with this because it is cheaper and most people are not going to know what is being sold to them unless they ask or know to ask. I am more on the parts side but there should be other techs on here that will chime in and give you other things to check if it still continues to happen.

i did use the original NGK spark plugs #fr2b-d. as of the grade of lp i will have to check into that. i have never rebuilt a Nikki vaporizer but have rebuilt many Impco vaporizers and regulators. i removed the Nikki vaporizer and pulled the back plate and there is a lot of sludge in there so im wiping it out and cleaning it up. are these vaporizers really that difficult to rebuild? doesn't seem like there is to much to it

What spark plugs did you use when you replaced them? Only use the original NGK spark plugs the heat range is higher than what other brands cross them to. The also have the last few threads removed so that they do not protrude through the head which will build up carbon on the threads if you use the wrong ones and most likely pull the threads out of the head when removing. Another issue is what grade of LP fuel you are using, most LP fuel suppliers, use the lower grades and the vaporizers do not like the sludge build up. If you are not familiar with rebuilding one of these vaporizers I would leave it to a pro or you can buy remanufactured versions from your local Cat dealer.