I may have not read this correctly but are your trailers self dumping? If it is a landscape or car hauler, no way am I letting someone else load it and I have to unload it by hand.
I have rented out my dump trailer before for weekend warriors, not contractors. $85 per day, they pick it up and they bring it back. Must have a 3/4 ton with a brake controller. Trailer comes back empty.
You should check with your insurance co and make sure you are covered for loss, damage, theft on someone else's job site. I know when renting larger equipment, I have to provide proof that I added to my policy during the rental period. Also have a contract written up similar to that of a rental yard for when you rent equipment. Make sure you have a clear and documented understanding of what is going on/in the trailer. I'm not sure about where you live, but I have to disclose when asbestos is in the trailer. It is also illegally to dump liquids, tires, batteries, etc. Around here, I am responsible when I dump. That is one of the reasons I have the weekend warriors bring it back empty. I was doing a private party rehab and the
homeowner kept throwing junk electronics, waste oil, old paint and other banned substances in my trailer. I kept pulling it out and putting it in his garage and told him I wasn't paying the fines for illegally dumping.
Lastly, check with local disposal companies to see what they are charging and price accordingly.