towing travel trailer with full holding tanks?

There are several Aps for smart phones. Here is one I found.
RVDUMPS
There are many others. I call ahead before I go out of my way to make sure they will allow me to use their dump facilities.

I see you are in Arizona. I have found free sites are rare in Arizona. Some of the large RV dealers such as in Phoenix allow free dumping.

I always dump ASAP. If you look at the black tank and how it is attached to the trailer I think you will agree to dump ASAP.