Hah! As everybody is saying, why would your brother want to start to change filters or do any troubleshooting at all if he doesn't own the coach?
Are we assuming that he is in fact keeping it for a day and test driving it for the purpose of buying it? Or is this some other type of scenario?
It could be a lot of things but I'm not sure if I was in your brother's shoes if I'd want to be doing any troubleshooting if I didn't own the coach.
One thing to quickly check is if the batteries are in good condition and don't need charging. Sometimes on a diesel, you can get a good healthy-sounding crank but there's just not enough juice to turn and start the engine. Perhaps the batteries just need some charging, terminals cleaned, water added, etc.?