RE: 2003, Ford F-250 with Triton V-10
My best advice is if you think you will be towing a big RV (30 foot plus) I would seriously look at diesels only. If however, you are thinking of a smaller RV, I don't think a V-10 would be too bad a choice. Fuel millage will be less with the V-10, but the initial cost of the V-10 will be less. I haven't heard too much about the V-10, because most serious RV'ers prefer the diesels to tow with.
If you look at diesels, stay away from the 6.0L Ford Power Stroke because they have had alot of problems, especially the 03/04 models.