You didn't mention if this is in storage or while boondocking (dry camping). There's a big difference in needs between the two. If this is just storage then the best thing you can do is disconnect the battery.
If you're boondocking, then there's lots of things you can do to reduce your drain. Most of them will be spread throughout the various threads about solar, but two of the biggest are:
1) change all your lights to LEDs.
2) if you have a dometic fridge, read the sticky at the top of the electrical forum about "batteries gone in 36h". Some of the fridges contain a heater strip that sucks back 2-3 amps of parasitic drain and that thread has information on how to disable it.
Lastly, as KyDan mentioned you'll need to figure out how much amperage you do draw. Once you have that, there's other threads on here that cover how to calculate the size of panel compared to your draw. It has several varying factors but as a general rule of thumb I found it worked out to needing a 40w panel per 1A of draw for a flat mounted panel in my area.
There's no use crying over spilled milk, unless it's on your keyboard.