No - voiding your warranty means that your warranty claim is denied; it does not necessarily mean that all warranty coverage is lost.
The more extensive the modification, the more likely you are to lose more warranty coverage. If you or a third party make extensive changes to the tractor's electrical system, you risk losing warranty coverage to the electrical system.
Example, you or a third party makes extensive changes to the wiring harness. Later your ECU experiences a catastrophic failure. The shop might find the source of the problem (not your modification), and you would be covered under warranty. Or - the shop may not find any particular cause, and they will conclude some sort of transient power fried it. The modification or a mistake during its installation could be the cause of the problem. Will they cover it under warranty? It is not so easy.
As to your tuned truck, it is unlikely that they would cover any engine problems that could arise from the tune under warranty.