Chessie do their absolute best to treat their animals well...
If these safaris were deemed to be unethical, they wouldn't exist at all... Quite simple really, as lobbyists will make a fuss (as has happened at Chessie before, hence the Gorilla enclosure getting major expansion recently) and get things sorted out...
Chessie have no reason to mistreat their animals, and I have no doubt they will have actually done research into the effects of not only diesel fumes (which using red diesel, will not be exceptionally high or damaging really) but noise, spacial issues, etc. The zoo is the most important asset to the park, and they would ensure the animal's health and safety moreso than anything else... Especially with an addition of this size...
Thing is with the zoos, is that they do a lot of work in keeping certain species in existance, just because the houses are clean and sterile does not mean the animals are being mistreated (indeed, as the houses are a place to do medical tests and what-not, you'd WANT it to be clean, tidy and sterile), similar goes for the safari, as the park will no doubt send the trucks around a LOT (possibly with food attached to allow animals to associate trucks with food and hence come closer) just so they can adapt to their existance...