Sell it and buy a new TT. GA law changed a few years ago. The tax is now charged up front when you buy a new vehicle. You don't pay the Ad Volerm any more after the initial tax, again on vehicles bought new since 2010 (I think, maybe it was 2011).
That being said, that does seem high. Our 2009 Motorhome was $350 this year. When we bought it new in 2009, it was $535 the first year. The value of the motorhome is based on the chassis value which I think was at least 3 times the value you said your trailer is valued at (Motorhome's in storage, can't check the registration). I'm sure Atlanta has a higher millage rate than Glynn County (east coast of GA), but that seems high. The last time we registered our 2006 HTT (last year) it was just under $100, I don't remember the value.
-Michael