I am unsure of others' experience, but our black tank valve started binding and wouldn't close when our Bambi was about three years old. When I replaced the valve, I found that the blade had jammed a little bit of toilet paper into the seal, preventing the valve from closing all the way (evidently, a common problem). Having already removed the old valve, I installed the new one I had just bought. I probably should have cleaned the old one and reused it, because the new valve never sealed well and has always leaked a little.
Our Bambi is about seven years old now, and it's time to replace the black tank valve again. The blade closes, but the initial leak has grown steadily worse; and there is always a little surprise waiting for me when I take the cover off to dump.
The gray tank valve has never leaked.
The valve assemblies are only a few dollars, and they are simple to change. However, it's a stinky, dirty job. If you can wait, it's not quite as bad in cool weather; and it really helps to flush the tank thoroughly to get as much of the waste water and smell out of the system as you can.
The new replacement valve that never sealed quite right was made by Camco. I am going to try a Valterra valve this time, to see if it seals any better. The OEM valve sealed OK, but it is the one that got clogged with toilet paper.
The black tank valve is 3" (I think); and the gray tank has a smaller opening (2"?).
I suspect that having an RV repair shop install a new valve would cost a lot more than what you might think, especially at the shop hourly rate. The actual job is simple; but the first time will probably take an hour or more, mostly because you have to keep washing your hands before you can wipe your nose.