I have a 1988 Dodge B250 rv conversion with a 318 engine 116000 kilometers. Fuel filter,fuel-pump and sock,spark plugs,wires,distributor cap and rotor, coil and magnetic coil in distributor all changed. No engine codes. Mechanic said good spark and fuel injectors. When i am on the highway and give it gas it is like a miss and no power. I noticed more when under load like going up hill. The vehicle will shake and loose some speed if i let up on fuel will go away. Mechanic said he took it for a long ride and no problem and said wait until it happens all the time. Tired of paying for parts and no change. Any help would be appreciated. 
Well to me that sounds like a classic sign of either the fuel filters clogged up or the fuel pump is wearing out. Because if you give it a bunch of gas and it won't accelerate but then you let up it does that shows that you don't have enough fuel pressure for full speed but enough for a lower speed
I had the fuel filter on the frame and changed and a new fuel pump and sock installed when i first noticed. I went for a 1000 kilometer trip and problem still there. After this i changed spark plugs,wires,distributor cap,rotor,coil and magnetic coil in distributor went for a 200 kilometer ride problem is still there and there are no codes. I have been recently told to change the oxygen sensor and have ordered one but wont change till spring as rv van parked for winter.