Ammonia build up can be a problem in tanks.

If they died of dehydration, I'd expect you should be able to tell by sunken eyes. I would probably switch out the food for a time, to see if that might be the cause.

Have you seen them just up and die, or do you walk in to find them dead?

Do they generally seem mellow, and easy-going or stressed out? It may be an old wives' tale, but supposedly rabbits can die simply from fear - gives them a heart attack. Anything that could scare them, like a loose snake?

If they were my rats, I'd open a few up myself, and if there was nothing obviously out of order, just curiosity would probably be enough to get me to spend the money for a necropsy. Nothing like a good medical mystery.

Good luck ending it, and please keep us posted.