Alex
You are asking alot from people if you think that many will book a place of accommodation, no matter how short a stay without full washing facilities.
It is pretty much a human right in the majority of the first world (and probably everywhere else) to be able to wash either before bed or in the morning. Without that ability, I would say 99% of people would keep away from your offering.
I would suggest that people on a one nights stay for business do not want to go to their job or meeting the next day without a shower and most certainly would not have the time to go down the local spa to get ready in the morning!
People on holiday, may be ok with it but after a long days travel or sightseeing in the sun, they want to wash off the days sweat. And whilst walking back down the hall in a shared dormitory situation is unthinkable for most… having to walk down the street in your towel to get back to your room, is not even a slight consideration.
If you want to rent your place, you have to offer a minimum of a bath or shower.
I dont know why you are adverse to putting either in to your property, or allowing them to share yours. But it will be the ONLY way to see even an inkling more bookings for your place. There is NO other way around it.
That really is the best help you can give yourself.
Ask yourself the question: would you book a place without a bath or shower, in ANY situation?