It depends on water quality..
standard Water heaters are designed to last 6-8 years. After that its a **** shoot..
A 10 year plus warranty water heater thats bought has two anodes in it..
My water heater thats on a well dont last 6 years. I can probably make it last longer by getting a powered anode. But I usually get one for free under warranty and then have to buy the next one.
If your heater is going on 15 years with no service of anode and flushing just leave it.. Once you touch it you probably will make it leak...
As far as RV heaters. Anodes are only needed if the heater is not drained. I drain mine after every trip...
Im a plumber 30 plus years...