Ditto^^^^^ 20 evaporate DT is normal for a good working residential or small commercial RTU s that have ~ 400 cfm/ton airflow but nothing is normal with RV and auto ACs. Low airflow causes the high DTs and ruins the efficiency. Low airflow/ton makes the system work on the latent load much more than the sensible load because of such a cold evap coil and EER, the original engineering term before SEER was drempt up, goes in the dumper. Cost and space restricts the proper design, big enough coils and fans, of a really good working AC in RVs and autos.
All of my RVs have had Dometic and after all the coil cleaning and fixing leaky ducts, dividers and casework they have 28-29 Evap DT. That's been the normal I've seen with these POS ACs. Craig