We ran into this problem on our last outing of the summer where we had no electrical hookup. We used battery power w/we thought the univolt, but upon returning home, we found something wrong. We had hooked up to shore power to clean up and found that the 120 outlets were working, but the lights did not. When we took trailer off shore power, the lights worked, but were very dim (because battery was just about done from our trip). We are sure the univolt must have quit sometime during the trip, so everything was then dependent on just the battery, which, by the way, will keep things like lights, water pump, etc. working w/o univolt, I just don't know how good it is for the battery to do that. Anyway, we are now planning to get the Intellipower w/Battery minder that others have said they replaced their Univolt with and like much better. There is a thread in here somewhere with a link to a fellow forum member who will give you a discount on his purchase site if you mention you are a forum member. Hope this helps.
PS - I went looking and found the thread w/the Intellipower info. It is on page 2 of this thread. converter