To setup the receiver for use with the Winegard dome I would recommend you set the receiver to 1-18" round. Have you verified that the dome itself has the proper dip switch settings for DirecTV (all up with #8 down)?
I assume you know you will not get HD but you will be able to use both tuners so be sure you have both cables from the antenna running to both inputs on the receiver.
Also be sure the receiver is set to multi-switch and not SWM. You will not be using a B-Band Converter. If you do not use BBC's in your home with that receiver (and you get HD at home) then your home system is setup for SWM and that will not work in an RV without the addition of the SWM8 hardware or ensuring you get the settings changed in the receiver off SWM and onto multi-switch.
What I post is my 2 cents and nothing more. Please don't read anything into my post that's not there. If you disagree, that's OK.
Can't we all just get along?