For SD content all you need is to lock in sat 101 to receive whatever programming you have subscribed to. The round 18" Directv dish can only acquire a single sat while the larger oval dish can attain all three sats which will provide all the HD/SD channels Directv carries.
We purchased the same slimline HD dish we have at home for RVing, acquiring all three sats for HD programming is not any different than acquiring just sat 101 as long as dish has been set to pointing parameters as outlined by receiver or whatever app is used.
Whether you lock in just sat 101 or all three Directv sats, you should always receive all the channels you have subscribed to just like at home. How the sat dish is pointed shouldn't increase or decrease the Directv programming you have under your current plan with the exception of free previews.
It is possible to have less channels when attempting to acquire all three satellites, which can occur when just 2 of the three available satellites are locked in. With Directv you always lock in sat 101, for HD sat dish just properly set tilt and other 2 sats should come right in.