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My guess is that the larger tank is an accumulator tank for your on board fresh water. This provides storage to reduce pump cycles and cushions the hammer from the pump starting and stopping.
The smaller one is probably for thermal expansion for the water heaters. Most systems do not require a thermal expansion tank because the pressure can expand into the fixture supply lines. If a check valve is installed in the water heaters cold water feed line, the pressure will not be able to expand back into the cold water lines resulting in the saftey valve opening. This is usually why a thermal expansion tank is needed. Jeff
Jeff and Lindsay
91 Marathon XL