Sorry for this but I need help with my 1992 K3500

At what altitude are you operating at? I seem to remember Sweden varies from sealevel to near 7000ft. That will wreak havoc with mpgs.
I haven't worked on those old TBIs much, but the ones I have seen, were running pretty rich, and had lazy timing.That's a pretty heavy truck for a 350 to have to tug up and down hills.
Those early systems were pretty basic. I don't remember them having a baro-sensor either.If your plugs came out black,that would kindof point to a no-baro system.I'm not sure about GMs, but some systems checked the Map at every start-up, for Baro compensation.So if you are going up and down, it might pay to perform a restart at every 1000 ft elevation change.
I would also go through the dizzy. I think 92s are still mechanical. If it has a vacuum canister on it, I would tune on that sucker quite a bit. I would also see if the engine will like the mechanical timing curve to come in a little earlier,and faster.You can do a lot with timing, for an engine that is running a lot of steady-state load and rpm.
Make sure the engine is running as warm as you dare and that the computer's temp-sensor is switching out of cold-start mode.If it has an 02 sensor,scan it and make sure it is cycling rich/lean.