I would be happy to help you make a wiring adapter to allow you to connect your 2013 Freightliner 122SD with a 7 pin round connector to your 2020 EZ4 triple axle trailer.
First you will need a female 7-pin round # PK11720 and female 7-way blade # PK12-707E. Since the 7-pin round has separate brake and turn signals but a 7-way blade has combined you will need the the 3-wire to 2-wire converter # HM48847 like you mentioned.
This will get connected to 7-way connector as follows:
Red wire... view full answer...