If you are calling it in "products_t.tpl then even {$products[product.fulldescr].descr|truncate:180:"...":true} is not correct. Your section loop should be named "product" so that would make the array pointer be [product].
So your existing code should actually be
Code:
{$products[product].descr|truncate:180:"...":true}
To get what you want, you need
Code:
{$products[product].fulldescr|truncate:180:"...":true}
If you do not want the description truncated to 180 characters, then simply use
Code:
{$products[product].fulldescr}