1995 Dodge Truck Losing Power

My truck after driving about 5 miles loses power. The engine doesn't labor or ping, the transmission works normally, but it won't go much above 40 no matter how much gas I give it. In one case, I was driving further than 5 miles and the truck continued to lose power over the next several miles. After a meeting of 1 hour, the truck started fine and run OK to home since I took back roads at about 40-45. In two other cases, I drove to the store (5 miles) and parked for about 15 minutes. When I started home, the truck started fine but i couldn't get it to go faster than 40.

I asked my dealer to check it out and they said my brakes had locked due to the wrong brake fliud having been put in (silicon based). I am the only one to put fluid in and as far as I know, have never used other than DOT 3/4. They want to change out everything from top to bottom for $2800 which is far more thant the truck is worth.

Is this a diagnosis I should trust or might it be something else? Additionally, do I really need to change out all my brake system or just change the fluid and do some other minor repairs. Please remember, while I am driving the truck for the first 5 miles, it works like it always did and the brakes work fine. I must admit that at least once I noticed a smell like something burning but thought it was antifreeze related and that system looked OK.