Are the lights bright when you are connected to shore power?
My RW is parked in my backyard, connected to 120. I use the porch light to let me know when I have lost power to the RW because after a couple days without power, the porch light becomes very dim. Once I fix the power to the RW, the light is immediately bright.
With all that said, it is likely your batteries. No charge or can't charge. If that is not the answer, I would measure the voltage at the light.
2015 38GK, Titan Brakes, H 114 Tires, IS
2015 Denali Dually