Option 1 Magnetics. Magnetics will be a problem because of the rear spare tire on the CRV being in the way. The bumper you can see and touch is mostly plastic so I'm not sure if there's enough metal for the magnetics to grab. You can probably stick the lights to the roof just not sure if 60 MPH+ winds when going down the road will push them around.
Option 2 Existing Lights. Taping into the installed lights are going to cause problems. The coach duplexes the brake and turn signal lights. Normally not a problem but if you add a supplemental brake to the CRV the CRV's internal braking indication will over ride the flashing turn signal indication. You can bypass this problem with one of
THESE or something similar which has a secondary effect of diode elimination. With the box you tap into both the brake light wire and each turn signal. The hard part is you'll have to figure out the wiring harness in the CRV to tap into the correct wires for each side of the CRV. Brakes, turn signals and night time running lights.
Option 3 New Lights. Drilling holes for secondary lights is pretty simple and direct unless you're uncomfortable permanently altering the lights. One down side is buying a large drill bit you'll probably never use again, unless you talk Effy out of his.
I did option 2 on an 05 CRV. Took a couple of hours but wasn't to hard working out the wires using a meter. Running the coach signal wire from under the dash all the way back was also simple. All the panels side just pop off and you can access the edge of the carpet to tuck the signal wire under. On my CRV the harness for the rear lights runs along the passenger side of the car and I tapped into the harness behind the rear panel where the jack is stored.
For the average shade tree mechanic/RV owner I would recommend option 3.