Well Steve stated 1.7 amp hours per day for a 12V
fan so 1.7AH X 12V
=20.4 watt hours per day, 0.85 watts per hour.
With that said it is more important to know why you want a solar panel for it.
Do you just want to keep the battery up while in storage?
If this is the case a 15-20 watt panel would be plenty, but may not recharge the battery if it's low.
Are you boondocking and don't want the extra draw to effect the batteries?
If this is the case you would be better off getting a more complete solar system for your entire trailer.