Looks like you have done your homework... pun intended. The recent one time 7 percent applies to motor vehicles only, not on trailers, makes registering in GA bearable. The no sales tax on used trailers from an individual still applies, but you still have to pay the annual property tax.
The $65K at 65 no GA income tax actually starts at $36K at 62. Unfortunately the $65K was supposed to go up to unlimited but they stopped it at $65K/person a couple years ago.
Plenty of Mail Forwarding companies in Atlanta, but you might have to physically go there the first time. You might want to pick one in a rural county where the tax rate is lower if you have a trailer. The mileage rate can vary from a low of 15 mils (times the 40 percent value) to over 30 in metro Atlanta. On an assessed value of $100,000 you pay a low of $15 time $40,000 = $600/year or over $1200 in Atlanta.