If I was 10 miles away I would hook up my trailer and go dump.
Me too! As great as the disposal tanks can be when your stuck without hookups, it's very heavy when full and really hard to lift and awkward to control when full, plus the thought of it opening up just about the time I got it to the bed of my truck is a scenario I would rather not experience.
I suppose if you were gonna be running into town etc where there is a dump station taking a tank full of either black or grey with you to dump would help to hold off making a, take the trailer run. But to make several trips to emtpy cause one tank full won't be enough to empty both of your trailer tanks so that would mean you would be making another run.