A tank will not save pump time. It moves the pump time from frequent pulses to a longer run less frequently. It also lets you flush the potty all night without the pump coming on, so it doesn't wake up DW/DH. If you think about it, every drop that comes out of a faucet had to go through the pump first, so the daily pump run time is the same.
That being said, an accumulator tank is one of the top 4 additions I have put on my RV. If I buy a new RV, I will immediately add another.
NOTE: Those who feel the need to point out that a shore connection provides water without it going through the pump, get a life.