The slight amount of toe in allows for axle and tire flex when moving down the road. When the trailer is moving forward the axle (or suspension in other vehicles) will spead (ever so slightly) so the tires will point to the outside. If the toe is set to inward slightly, (toe in) the spread will create a zero toe and there will be no adverse tire wear. At least that is the goal.
So a unit with lots of use (and I consider 30 years to be lots) with no adverse tire wear I would count my blessings and move on to something else that is visabily wearing.
BTW the same holds true for camber, however the issue there is loading of the unit and the dynamic forces applied when this load is towed.
1966 Mercury Park Lane 4 DR Breezeway 410 4V, C-6, 2.80 - Streamless.
1966 Lincoln 4 door Convertible 462 4V 1971 Ford LTD Convertible 429 4V Phoenix ~ Yeah it's hot however it's a dry heat!