What year model TT do you have? Was it the 30Amp fuse at the battery box that you removed? How many wires do you have connected to the TT's battery? 1 Negative (to frame) one Positive to TT? Maybe the emergency break-away switch power? How many batteries?
Was the battery FULLY charged when you put the TT in storage? You also have to understand that a 15Watt SOLAR panel will only put out a little over 1 Amp for an hour or 2 during the day, after that it is a lot less than 1 amp. Is the SOLAR panel exposed to FULL sun all day while in storage?
When you put the new battery in, and with the FUSE still removed, check to see if you have power in the TT. If the battery was not fully topped off before letting the SOLAR charger do its work, there may not have been enough SOLAR power to get it to full charge.
I normally do not recommend less than 50 watts of SOLAR to keep the battery charged. Due to the winter sun angle and lower amount of sun power.
RVing with SOLAR