What I have done in the past was to run to Walmart or the local auto parts house and pick up a interior/exterior cig lighter and mounted it in the vehicle out of sight. The last time I mounted it under the drivers seat where is was easy to get to.
I would wire the cig light up direct to the battery with an inline fuse for safety. The reason I used an interior/exterior cig lighter is that they come with a spring loaded door so that when not in use the door remains closed so there is no worry of anything getting into the lighter.
Total DIY cost under $10
2018 Berkshire XL 40B
Past RVs: National Pacifica 40', Kenworth T300 Toterhome, Jayco Class C 32'. American Eagle 40',
American Eagle 40', Bounder 38' tag axle