I don't think this is a good use-case for a multiplier. The problem is to display a value in decimal format. For this, a small FSM with an adder/subtractor seems correct. The FSM would accumulate 100 (just a guess) samples. After getting 100 samples, the fsm would try to subtract 1V. if there are 4095 counts per 5V, then there 819 counts per 1V. Because the samples have been accumulated 100 times, 1V would be 81900 in the accumulated version. With each subtraction, the unit place of a temporary output value is incremented. this continues until the accumulator is lower than 81900. At that point, 8190 is subtracted until accumulator is less than 8190. the 0.1's place is determined based on the number of times 8190 is subtracted. The process repeats for 819 to get the .01's place. After this, the temporary output value is moved to the output -- all digits update at the same time.
There are other variations on this. This one is easy to understand. The sampling rate should make it clear if accumulating 100 samples is acceptable, or if the input should be multiplied by 10 or 100.