Several years ago (four or five) there was a discussion on the forum about weight distribution on conventional trailer toyhaulers. It centered around the tongue weight being much too light when a heavy load (but within the weight limit) being loaded. One owner had to install an additional water tank under the bed and one resorted to (or was considering) adding sand bags as ballast.
I haven't heard of those kinds of problems since then, so I'd think the manufacturers have resolved that kind of problem. Possibly they haven't and you're problem is similar.
If you do a forum search for "sand bags" or similar phrases you'll find more information. Here is one "old thread" :
http://www.keystonerv.org/forums/sho...highlight=sand (There's some good information in post #5 of that thread)