Like this one?

Found this on tuning it on a Dodge forum:
Adjusting GAIN
NOTE: This should only be performed in a traffic free
environment at speeds of approximately 20–25 mph
(30–40 km/h).
1. Make sure the trailer brakes are in good working
condition, functioning normally, and properly adjusted.
See your trailer dealer if necessary.
2. Hook up the trailer and make the electrical connections
according to the trailer manufacturer’s instructions.
3. When a trailer with electric brakes is plugged in, the
GAIN setting will illuminate.
4. Use the GAIN adjustment (+/-) buttons to increase or
decrease the GAIN setting to the desired starting point. A
GAIN setting of 6 is a good starting point for heavier
loads.
5. In a traffic-free environment, tow the trailer on a dry,
level surface at a speed of 20–25 mph (30–40 km/h) and
squeeze the manual brake control lever completely.
6. If the trailer wheels lockup (indicated by squealing
tires), reduce the GAIN setting; if the trailer wheels turn
freely, increase the GAIN setting.
Repeat Steps 5 and 6 until the GAIN setting is at a point
just below trailer wheel lockup. If towing a heavier
trailer, trailer wheel lockup may not be attainable even
with the maximum GAIN setting of 10.