The water tank on our Surveyor 263 is situated right above the axles. When the fresh water tank is full, the cover on the bottom of the camper will lay on the axles (torsion axles, so they don't move). I can still squeeze my fingers between the cover and the axles, so I haven't worried about it too much. So far, so good. My tires wear good, so there appears to be no axle bending from the tank weight.
But, your situation seems a bit different than mine. Any extra support should be a good idea.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263