I hope you have actually been inside a 13' FGRV with a bath/shower combo. They are rare, perhaps because they aren't as popular as one may think. Bottom line, they are very crowded and it's almost always mandatory to convert the bed back to a dinette every day. With the front couch this isn't always necessary.
But, as far as price, that's a tough one. We sold our 2004 13' Scamp/with loo, last year for just over $10k in California. In addition, condition is everything.
With larger Casita's available for under 10k right now, except on the west coast, I would price Scamps at something below that number.
Sooooo, I would say that something around $6500 is a "good" price, but you might not be able to find one for that price, not at least until mid winter.
Be sure to remind the seller that the season is OVER, and prices will fall
on a daily basis as the cold/wet weather starts.