Does not wash very well?

Sears dishwasher Model 665.17519
I think the problem might be too low water level. It uses only 1.25 gals of water per cycle. That's a total of 5 gal for the 4 cycles. The book says it should use 7 to 10 gal. The water level does not come up to the heater element and just barely reaches the lever overflow float thing. Also, the water level only slightly covers the very fine white screen material at the top of the Accumulator Assembly.

Do you agree that it could be a low water problem. If so, how do I fix it.
Where is the water level sensor and how do I buy another and replace it?
Can you provide me with replacement instructions?

Otherwise, what do you think the problem really is??