I think the rep at DirecTV was confused. She may have been giving you the answer for a residential DNS account which they do offer in areas where no local channels are provided. SD or HD in that case is based on what the market provides in their area.
1. ask for an RV account. Use your mailing address. I gave them a credit card for billing. For an RV account they don't give you DNS based on an address. Therefore you should be able to get HD DNS in an RV.
2. submit paperwork for DNS channels.
3. get HD equipment
Keep us posted.