I've got one of the fiberglass ones. I think it's from these guys:
RVFLAGPOLEGUYS: #1 RV FLAGPOLE GUYS
I got the 22' one. I have a ladder on the back so mount mine there. The bottom cup I put about 7' above ground. The top bracket is near the top of my ladder. I know, you don't have a ladder, yet. You might want to consider putting a ladder on just to hold the flagpole? I think I wouldn't want to mount a 22' pole to the mirror. With two flags flying and a bit of a breeze it has some torque to it. The top bracket on mine sometimes twisted a bit in the wind. So I drilled a couple holes through mine and pop rivited it to the ladder so it won't twist. Then for extra I climb up there and put a short bungy cord around it and the ladder to prevent it from popping out (it never has but I'd hate for it to spring out and land on my neighbor's rv).
Here's a shot:
I've been happy with the gray fiberglass pole. A few time's it's slipped down a section. I talked to the guys about it at the last rv show and they gave me a pin kit that I haven't installed yet. You extend the pole and drill a hole at each overlap. Then they have these neat pins that fit in to assure it won't slip down. I'll drill the holes soon.
What about a base type tube that fits into a trailer hitch receiver? Would that work for you? I think if you get it out about 2' from the back of the coach I don't think it would hit up top. I think those kind that you drive the tire onto are pretty low and don't stick out very far so might be more likely to hit up top when windy?