How important is tongue weight with lots of weight in bed

You need the correct weigh distribution for the trailer, or you are asking for a wild ride or worse.

I read about one guy who loaded the same ATVs in his trailer all the time, and had actually had the trailer weighed and shifted them a bit, to get the right distribution. Then he painted around the tires (or something like that) so that he'd know where to park them each time. Of course, if he added more stuff, he had to re-calculate, but he at least had a good starting point.