I’m not paying to fill the tank if they are hauling their own horse, too, and I wouldn’t let a guest fill mine. Their 75 gallons of diesel will not put a dent in my 36k truck and 46K trailer LOL. A tank of diesel would be charity if I’m a guest I gladly offer gas money and pick out the trailer, don’t wipe cheeto dust on the seats LOL, and clean up after myself.
I hate working out of someone else’s trailer, and I don’t really like taking other folk’s horses to shows- I do my thing my way and that’s how I roll.
Drive yourself. Just tell them both you decided to drive yourself. Period, full stop.