chas,
Your coach has two separate 12V lighting systems. The exterior lights, marker,
brake, backup, turn signals get their voltage directly from the tow vehicle.
The Univolt is part of the interior 12V system. It along with the coach battery
provide power to the interior lights and appliances.
As with any DC lighting system there are two parts to them. You can have the best,
hottest battery feeding the lights but if the ground is corroded or loose then the
lights will not shine brightly. It takes surprisingly little corrosion to cause
problems. An old trick of the trade is to spray diluted vinegar into the light
sockets. The vinegar is the chemical opposite of corrosion and will neutralize it.
If all the lights are similarly dim it suggest the tow vehicle/trailer connection
is defective. It may be necessary to break open the socket and plug to clean up the
terminals.
Charlie