New to towing vehicle with E-Z Pass question

I to have just one EZ pass transponder and move it from vehicle to vehicle---I sometimes tow a 2 axel trailer and sometimes one with 3 axels. I have been all over the country and was always charged correctly except some times the 3 axel will be only charged as 2.......$36.00 to cross that bridge on 301 from MD to VA is pretty stout!