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.
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Chap , DW Joy, and Fur Baby Sango
2017 F350 Lariat CCSB, SRW, 4x4, 6.7 PS
2017 Grand Design Reflection 337RLS