No, you can take a D12 "SD receiver" with you (buy one on eBay that is new - about $79.00) and it will cost you an extra $5.99/mo (or $6.99/month) They don't care where you place the receiver. If you have a HD receiver at home, you wouldn't be able to use that afaik.
No special "mobile units" needed
Here is a D12 Receiver for $99.00
D12 DirecTV Standard Def Box & RemoteI see where you're reading the bunk DirecTV says about their "mobile packages" Ignore what you read there and do as I mentioned.
DirecTV Bunk & Nonsense