Hard choice. Cost to replace is high but you'll have to discount the rig heavily as compared to other comparable rigs to get someone to buy without one. You might find a buyer who will take it as is but a lot more will walk away not wanting to put the effort into an older coach even with the discounted price.
At this time I would wait. If you get a lot of lookers but no reasonable offers then consider installing the correct generator model for the coach and reevaluating the sale price accordingly. You won't recoup the cost in material or time but it will make the coach more marketable. Call it curb appear.
As a few have mentioned, I wouldn't even consider a coach without.
