If you wish to not use copper vouchers for it, you can grind it the old way. Fields of valor give less rewards than Grounds that I have noticed.
http://wiki.ffxiclopedia.org/wiki/Grounds_Tome
Here is a handy list of different grounds tomes regimens and the rewards for them. There is a range because as you complete regimens over and over, you get more rewards. For instance, if you do the crawlers nest #7 several times over, you'll get 438 tabs on completion (with the game day cap in consideration as posted above though I haven't noticed that before). Your first completion will be only 219 tabs however. Since you get some very potent buffs from prowess on completion, the killing goes quicker as you stay longer too. I grinded 90 to 99 thf by just staying in one area doing the same regimen over and over and over with stronger and stronger buffs. That is probably the best way to grind tabs traditionally.

For just one copper voucher though I believe you can get both trusts as they are 300 each and 1 voucher gets you 1000 tabs, and it would be much faster if that is your only goal.