I had followed several FAQs about the issue that all were 404 at DTV, but AT&T owns them now and here's what I found from them about DNS:
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DNS for your RV, car or boat
Certain types of vehicles are eligible for DNS without requiring home address eligibility. For eligible vehicles, see item No. 11 of the FCC Satellite Home Viewer Improvement Act fact sheet. Boats and other marine vessels do not qualify for this special consideration.
Note: A separate monthly package is now required to enjoy DNS in your vehicle, instead of a smaller mirroring fee that used to be allowed.
To add DNS to your mobile service
Send a copy of your vehicle registration, a completed Mobile Vehicle Affidavit Declaration of Intent form, and a copy of your commercial driver's license (if requesting service for a commercial vehicle) to the address listed below.
DIRECTV
Attn: Mobile Network Services
P.O. Box 6550
Greenwood Village, CO 80155
It will take 11 to 14 days after receiving your paperwork to verify the information and activate your DNS programming. Your dish must be pointed to the satellite located 101 degrees West longitude at the time we activate DNS service for your system. If you are not receiving DNS programming after 14 days, make sure your dish is pointed to the correct position and then call us to request activation.
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I think the problem was that I didn't activate the satellite for several weeks after I sent in the form and they say they'll look for my dish in 14 days.