Faulty fuel injectors

Hi, I have a '99 Eurovan camper that seems to be blowing fuel injectors. Here’s the history:

1. One day the carbon monoxide alarm went off and I realized that the deep cell battery had discharged. I drove the van around and the battery charged but after being parked for a while, the alarm sounded again. At the same time, the check engine light came on and the van started driving rough.

2. I brought the van to the mechanic and the scan showed that fuel injector no. 5

had an electrical fault. The mechanic also determined that the alternator was

bad. He replaced the alternator and fuel injector no. 5 and it seemed fine for

a week of around-town driving.

3. I drove two hours and the check engine light came on and ran rough again.

The scan at the closest VW dealer showed that injector no. 4 was bad. I drove

back to the original mechanic and by the time I got there, the scan showed that

No. 2 had gone bad also.

Replacing fuel injectors in the van is very expensive and I want to make sure that whatever is causing them to malfunction is fixed but the two mechanics don’t know what the problem could be. I don’t believe that it’s a random occurance that fuel injectors are all malfunctioning around the same time. Any ideas??