While I think it is admirable for the OP to want to help out the widow of his late friend, that is probably not going to be enough. Selling any Class A, especially a higher end model, even though it is a 1999 model, is going to take someone with specialized knowledge.
Any prospective buyer is going to have a bunch of questions about the bus, they are going to want detailed info about the different systems, water, electric, generator, heating, cooling, cooking, refrigerator, etc. To get the widow a fair/reasonable price for her bus, the person handling the sale is going to have to be knowledgeable about Prevost buses and their converters.
Whine has given you one name, also consider putting the widow in touch with PPLmotorhomes.com in Houston, Prevost-stuff.com on line, and any other dealers that sell Class A units. I would also check with Berryland Rv in Louisiana on Interstate 10, to see if they handle consignments of buses.
To really help the widow out, I would strongly recommend the OP advise her to turn the matter over to a professional to get it sold. The OP could stay involved by helping her screen prospective dealers/brokers but then let the professionals do their job. If the OP owned a Prevost himself or was intimately acquainted with Prevost conversions, then that would be a different story, but that isn't indicated to be the case.
joe b.
Stuart Florida
Formerly of Colorado and Alaska
2016 Fleetwood Flair 31 B Class A w/bunks
www.picturetrail.com/jbpacooper
Alaska-Colorado and other Trips posted
"Without challenge, adventure is impossible".