Water pump going?

It can also be an air leak along the parting flange of the pump. Mine leaked there. After removing the bolts and cleaning the diaphragm several times and torquing the bolts and buying a new pump I finally figured out that the corrosion along the parting plane was preventing a seal. I put Shoe Goo along the parting plane and re torqued the bolts and the pump worked fine. Look along that plane for any leakage or corrosion.

Sounds to me like an air leak in the supply line, a new pump is likely to do the same.

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Check with the manufacturer's web site with the model number and see what they offer
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They offer a new pump for $60
I'd bet its a restriction on either side of the pump. After all, how can even a wore out pump loose its prime when it is gravity fed w/ even 1/2 a tank of water pressure above it ??

Most pumps have a filter screen on the pump end , this might be plugged, but 20 years is a long time and maybe time for a replacement pump. You might be able to purchase a rebuilt kit too which is a replacement for the head section, I would check for a filter screen plugged first.
Check with the manufacturer's web site with the model number and see what they offer.

Sounds like a restriction. Does your pump have a screen style filter on the tank side?
There also could be debris in the opening to the tank.

Water pumps are one of the least expensive items in an RV - techs won't even bother to work on them because replacement is less expensive than fixing. Time for a change.

if the motor sounds good you may just want to get a replacement pump head kit that just bolts on the motor, but after 20 years and the difference in price for a new pump, I would get a new pump and by the way, a new pump installation is so much easier.

It sounds to me more like an air leak issue, which may possibly be the pump itself but more likely (I suspect) is a problem with the plumbing connections to the suction side of the pump. It stands a good chance of being an easy fix, depending on access and the ease with which the problem can be found.

20 yrs. is a pretty good run....they don't last forever.

And for $60 you can have a brand new one LINK