Sounds like your block heater is doing just fine. At best they will only speed up you heat up time, I don't think they will ever take the hand off the peg. As for the farm Diesel be careful. Modern Diesels are awfully sensitive and not all died Diesels are created equally. You could try a weak mixture of farm to pump Diesel and closely monitor your truck for differences. In the end do you think you will save enough to cover the cost of potential damage to your truck.