If you take the RP tech job, expect a delay getting into Ops - they will expect you to complete RP quals first.
We have a former Chem tech in the Ctrl Rm now - but it took her a hard push to get into Ops, be / qualify AO, and do license school. She has a degree in Chem, and is eligible for SRO school in the future.
Starting in another Dept adds 3-4 years to your advancement track IMHO. Doesn't kill it, and the experience could add something to your knowledge / experience. The same would apply if you started in Maintenance - you would still have to transfer to Ops.
As far as what an RP tech does - it's Radiation Protection. Surveys, exposure estimates, emergency response training, some instrumentation. Training (and yes, you will have initial training) and qualification takes 6 months - 1 year. Then most likely on shift (at least at my plant). It is different than Navy Radcon - at least to those ELTs I've talked to.
Good luck.