Hi,
Ours are the same way. Changing bulbs didn't help, so it must be the ballast. I'm considering replacing them with these
LED replacements. I'm not endorsing this site as I haven't tried their product yet. These DO NOT USE the existing ballast and it must be by-passed or removed. The video on the link explains the process.
I also know there are cheaper alternatives, but you have to be careful with the voltage. The bulbs I linked to have a built-in voltage regulator (they are designed for 12V or 24V systems). From what I've read, some that are designed for "12V only" have a shortened life during the higher voltage battery charging cycles of our inverters. There are several posts on here of people who have replaced their fluorescent bulbs with some sort of LED system.
~Rick