Yes, sometimes an "idiot light" will malfunction and give a false indication. Of course it can also malfuncion the opposite way and give no indication when there is zero or low oil pressure.
I would not take the owners word for this. Have the coach checked by a qualified diesel mechanic. You can offer this at your expense, but if the pressure is indeed low, I'd pass on it. If it is a malfunctioning light, have the owner make the repair and reimburse you or drop the price to allow for this.
Amateur Radio Operator (KE5DFR)|Full-Time! - 2012 6.7L Ford Crew Cab Dually -2013 HitchHiker Champagne 38RLRSB - Currently FOR SALE Travel with one Standard Schnauzer and one small Timneh African Gray Parrot, retired mechanical engineer