Wow - thanks for all of the replies!
I went and checked some of the things you all mentioned... here is some more info.
Four people in the house - one big whirlpool tub, one regular tub/shower, and one freestanding shower stall.
When the big tub is used, we usually run out of hot during the 2nd or 3rd shower at or near the same time as the whirlpool was used.
We've pretty much have had this issue the whole time, but the kids were young back then... now that they're getting older, it's longer showers/baths & it's also harder to schedule their bathing time in order to spread out the usage.
I checked the hot water tank... looks like it's plumbed correctly - the inputs at the top are labelled hot/cold & the piping seems right.
I flushed it for sediment just a couple of years ago when I needed to replace the controller board or some such thing... it was originally installed in 99, so it's ~8 years old.
The model is Rheem "21VP50E - 1 A". It's a 50g gas model with this info on the label:
input btuh 40,000 Manifold 4.0 Cap 1st hour 72g
Do you think it's undersized for the load we have with the whirlpool tub?
I was leaning towards the extra electric tank since I can do that for a couple hundred $$$... I think a tankless that can handle what we need would be a grand or two, no?
Thanks again!