I'd ditch the RD board. I've had very bad luck with them, including non working displays, and schematics/theory of operation are limited. Better off repairing the original boards if possible.

Look for the contact tabs of the battery holder to be broken off. It is easy to overlook. With your multimeter check for the battery voltage at pin 22 of the 5101 RAM chip.

Make sure you fire it up with coin door open one time so that the 5101 is writable and thusly is cleared. (Or at least on my Gorgar I seem to get stuck in audits after swapping batteries until I open the door once)

Make sure you are not mixing up the two four pin plugs. One has the display blanking signal. I wouldn't think you are getting this on wrong if the originals work.

Displays have digit strobes and segment data. For everything to be blank, I would go to the strobes first. Check display strobes at 1J7 and 1J6 when the displays would be lit up (go into display test if you can blind). If the signal is solid high, check the PIA pins that control the decoder chip PA0 to PA3. The decoder is supposed to be always enabled. Looks like rottendog might be using a mod board to replace the obsolete decoder. That mod board might be a good place for things to go wrong at. If the PIA signals look good, maybe find a real 74154 , 74LS154 or 74HCT154. Probably don't want 74HC because there are not pull ups, but it would probably work.