You have a good question, I do not know an official Sunline answer, but I will give you my opinion.
Sunline was good to us; they created a heavy cage under the tank to allow us to carry freshwater while traveling to camp. I'm not sure what years this heavy cage started, but I know it was from 2003 until the end.
The grey and black tanks are a different story. Those thermoformed tanks bellow out on the bottom when filled; no extra support is under them. The bellowing tank bottom can rock/flex the discharge pipe when the bottom of the tank moves and crack the pipe at the tank, given enough flexing. The grey pipes have cracked at the tanks before at the tank. I have repaired 2 Sunlines for friends with cracked tank fittings at the tank, and at least one other member posted semi-recently with a cracked discharge pipe at the tank.
Here is my view, seeing how much those tanks bellow out is a concern, not so much for in-camp to the dump station going slow, but for highway travel. If you hit a pothole or other small bump at speed, the tank bottom will flex, and the tank and discharge pipe will be affected negatively.
If you are forced to tow down the road with a full tank, try and go slow. I would not do this unless forced into it with no other options.
Here is an option that can help reduce possible damage. Use a portable blue tote (any brand) and drain some of the water out of the grey tank daily that you can lift into the truck bed to the area you can dump it. I do this on the grey water when boondocking beyond four days on extended trips until we get to the camp dump station. I have an 11-gallon tank I can lift into the truck bed. It is about all I can lift too. I also did it on the black to remove some volume, not the whole tank when we camped longer than 8 days. Sometimes I have to make 2 trips on the same day on the grey, 11 gallons at a time if I missed a day due to bad weather or we took a day trip.
If you plan on a full grey and half full black towing at speed, heads up on what that amount of weight will do to your loaded tongue weight. It may go very low or high, pending on where the tanks are located. As a rule of thumb, 45 mph is a starting point when an unbalanced trailer can start into a sway event.
Several years ago, EMD_Driver had to tow a short way from camp to a local dump station as the one at camp was not working etc. I recall him saying his F250 was really squirrelly due to the imbalance with the loaded tanks. He did not report tank damage but did towing issues.
I hope this helps.
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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