are you absolutely certain it is not showing a code on the dash display?
it's very unusual for these trucks to not show a code, out of the components you have worked on or mentioned, if there had been anything wrong with them there should have been an associated code displaying on the dash. (unless your dash display is faulty?) does the display show anything? HM?
is the fuel system getting fuel? when you turn on the lp tank you have pressure at the lp vaporizer? pressure at the mixer/injector ports when you turn ignition on?
even with a situation like that you would be getting a low fuel pressure code or something.
not showing codes and not starting is very strange...
i'll check my service grams and see if there are any covering this scenario....
stay tuned...
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update: found one service bulletin about hard starting and poor performance and it references the fuel filter in the mixer/injector body. Depending on the year of mfg this filter is located either under a small plate on top of the injector body (2 screws holding it on) or underneath the map sensor where it mounts. It is a mesh type filter screen that can get clogged up. Later models moved this filter screen to just inside the fuel hose elbow where it mounts to the injector body, 2 screws hold that on, remove the elbow and the screen is just inside that hole. it will have a small clip holding it in. DO NOT poke a hole in it, carefully remove it and clean it or replace it.
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second update:
found another bulletin about no start no codes dealing with the LP cut off solenoid on the vaporizer. This lock off has 2 issues on record of failing by staying open when the key is off and the second issue is staying closed when the key is on. This issue is caused by tar buildup inside the vaporizer and getting into the solenoid lock off and causing it to stick either open or closed either flooding the fuel system or not allowing fuel to pass. If it's hung open you would smell LP gas even with the engine not running, the other case would be a no start scenario and no LP gas smell at all.
Remove the solenoid assy and check the filter, clean or replace it, clean the valve body, plunger return spring and plunger valve. You can hook up manually to a 12v power source to make sure the solenoid is activating to check the coil.
DO NOT use sand paper or emory cloth to clean it, use a non caustic solvent only.
if you need to replace the filter the cat part number for it is 91A65-00611, the oring part numbers are 91A65-00612 large oring for cover, 91A65-00615 small oring between filter and valve body, and 91A65-00616 small oring between lock off coil and body.