Thanks will check ,seams like it has good power in neutral

First is the check engine light on if so you need to find out what codes are stored.the 7series trucks had different emission systems in some trucks some had drive by wire some did not there were different version of the truck.it went by the series number l not sure what they used in your truck.as for dropping into low gear this should not happen you could have a trans problem or maybe it feels like that because it is loosing power from motor does it have power in neutral if not l would look at possible propane problem the converters would gum up with the tar from the propane and stick the seats in side the regulator this could cause low power also if uses injector on side of carb make sure you here it clicking.on the converter so had a drain to drain the tar but l never got one to drain it is to thick to drain had to rebuild or buy new.check timing. Also it is possible if it has power in neutral then when dropping in gear you are loosing it it could have a bad toque converter in trans hope this helps