I have an Epson printer. I am familiar with Epson iPrint (an Epson app for iPhone/iPad), but I am not familiar with Epson Direct. So I cannot tell you what Epson Direct can or cannot do.
You could print from an iPad to your Epson printer if you had a WiFi hotspot in your RV, and had the iPad as well as Epson printer connected to the hotspot. The hotspot would be provided by your cellular carrier, and would not be free.
UPDATE - I now see an Epson XP420 is not an AIrPrint capable printer. Therefore, to print to it from an iPad, you would need to use Epson iPrint, which has much more limited capability than Apple''s AirPrint. I do not know about Epson Direct, nor can I tell you whether you can print from your Mac (you definitely could print from your Mac if the printer were AirPrint capable).