I bought a set of replacement 7/16" fibreglass tent poles (about 8 1/2' assembled length), which were stiff enough to stand vertical holding the weight of a flag.  To the backside of the trailer's tongue jack, I zip-tied a 18" length of 3/4" (inner) poly pipe.

When it comes time to setup, I slip together the fibreglass poles, attach a flag to the top with binder clips and slip the bottom into the pipe for support.

With the height of the tongue, the top of the flag is about 10' in the air.  Its located at the front of the trailer, which is generally the most visible spot from the road.  And, given the curve of the roof, my flag stands proud and doesn't slap against the trailer.   For me, its just about the right solution.

But, if you're looking for something a little bigger, Camping World carries a telescoping pole with a base plate that you slip under a tire for stability.