Today I noticed my tucks RPMs are lower than normal - both while driving and idling. I would say that it normally runs about 2000-2500 when driving at a normal speed, but today it was only about 1500-2000 (idling normally about 1000, now about 500 - and still stays running). It also seems to be accelerating a little slower than normal. If I really press on the gas it seems to be fine, accelerates without a problem.

My check engine light is on, and there are no abnormal sounds or smells coming from the truck.

Any ideas as to what may be causing this?

Capless fuel filler. After filling tank engine light and check fuel inlet light came on. Now diagnostic test P06B8 code comes up that says transmission problem. When purchased I received a letter from Ford telling me transmission needed to be replaced but when I brought to dealer as a recall they simply reprogrammed the system. This was done twice. Now stuck with "on" engine light and vehicle will not pass inspection. Not sure where to go from here but I am checking Ford tech bulletin. Last Ford ever though!