I found a very good Used Motor Home Checklist on the internet. No idea who created it.
I used it in 2007 when I bought my Safari.
If you can't open a pdf file let me know and I'll post a Word document version.
I recommend that first you do the checklist, then if satisfied look at maintenance records, then if still satisfied take it to a diesel shop and have them do a mechanical check. Might cost several hundred dollars but well worth it considering the purchase cost.
I'd also recommend checking the NADA pricing WITHOUT options. It can give you a ball park figure, even though its based on a depreciation formula rather than sales data.