This is a great question that I have been asked a few times. So much will depend on how each barn owner sets up their facility and what they offer as "Extras." At my barn our boarders are allowed to keep their trailers on our property as long as space is available. I do not charge for this service but there are many barns that charge a fee. I will allow my boarders to take hay for the hay box and extra for overnight or horse shows as long as they are frugal with it and don't waste it. If I find that the hay is being wasted then I will consider charging a fee for the wasted hay. I haven't had much of a problem with this at all and it really depends on hay prices for each year. If hay prices were to skyrocket then I would probably add a fee for extra hay that is taken over and above the normal amount we feed for horse shows and trail rides overnight. There are barns that charge a fee for hay that is taken to horse shows or trail rides and that is up to the discretion of the barn owner on how they want to run this part of their business. One thing to think about is that people will often feed much more when at horse shows and on overnights than what would be normal feeding back home at the barn and the barn owner should not have to eat the cost for the extra hay that is fed and many times wasted.
I do not provide any bedding at all and I don't know of any barns that do in our area. Usually that is something that you will need to purchase on your own. In all the years of running our barn, I have only had one or two people ask to hose out the inside of their horse trailer because it was dirty. We have well water and there is no monthly water bill so that is not a problem at this time but if a barn gets their water from city hookup and they have a monthly bill then it will probably change things and at that point a fee will be added. I have found that most people take their trailers home to wash.
Every barn owner will set up this part of their business differently and there is not a right or wrong way when it comes to these kinds of extras that are not part of the normal boarding operation. Often horse trailers and all that goes with it are extras and are usually handled separately.
The best thing you can do as a boarder (and your barn owner will love you for it) is to ask before you do any of these things. The most frustrating thing for a barn owner is when a boarder just assumes that it is okay to take hay, bedding or decided to wash the horse trailer using the barn owner's water without asking first. Your barn owner should provide a list of things you can do and not do as it relates to horse trailers and traveling and any cost or fees should be attached.
Thank you for being a conscientious boarder!