You can get a step up converter board on Ebay for around $5 shipping included. Here is an example.
https://www.ebay.ca/itm/DC-DC-Digita...item4d8fd6304a
I use them in all my antiques and they work pretty good although you end up replacing them every so often so buy an extra. I always put in a cigarette lighter outlet which is really cheap as well. That way you can charge your phone etc. on tours and run a dash camera. Just disguise it or put it in a hidden spot.