I agree with the last post if there is more than one person useing it for more than a few days you may fill it up, the main reason I did it this way was I already had the portable toilet and I camp alone and it solved part of the problem of dumping the toilet.
If I were building a trailer from scratch I would build a better bathroom toilet and holding tank like you would find in a factory type trailer,you would spent alot more but worth it to me.