Good afternoon,
The RV042 is capable of rules that map multiple public IP addresses in a range to multiple private IP addresses in a range. This should be available once you check the "enable" checkbox on your RV042.
Ensure you have the newest firmware of 1.3.12.19-tm
I will try to locate a RV042 and upload screenshots of what this looks like. I do not see that the documentation specifcally state that one-to-many is available, but that type of option is available under the one-to-one section.
I hope you found this helpful.
Thanks,
-Lindsey