A magnet mount on the roof of a motor home would be way way too high in most
situations...it could get swiped off while going under over passes or low
level bridges.
My recommendation is a mirror arm mounted antenna, such as the wilson 1000,
or 5000.
This is a great antenna for the CB band.  You could also consider a fiber
glass firestick or something similar to put on the mirror.
If you do install on the mirror, try to make sure there is a ground path to
the vehicle chassis.  Many RV's have allot of fiber glass in them and
plastic, and the mirrors are not always grounded properly.
The CB won't work at all without a properly grounded antenna.
I know its just for local info, but if you can't be heard past the end of
the block, there is no point.
Dual antennas such as those seen on big trucks are not any more efficient
and do not perform any better then a good quality single antenna.
 firestick, hussler and wilson are the big names and good quality products
in the market right now.  I believe larson also has some CB antennas as do
many of the amateur antenna manufacturers.
Of course, the best mobile antenna for 11 meters is the 102 inch steel whip,
but these can be a pain to install properly and unless you can install it
high up on the rear or side of the vehicle, it won't really work.
The best configuration would be a 102 inch steel whip on the front or rear
or side of the RV with about half the antenna above the roof line, and at
least 4 inches away from the metal sides, closer if they are composit or
fiber glass panels.
The 102 inch whip is also one of the cheapest antennas, usually running
around $20 for the whip and another 20 for the mounting hardware.
Of course, with mounting on the rear, you are looking at a relatively long
run of mobile coax and a bit of a head ache running it.
Again, the next best would be a wilson 1000 mounted on the mirror arm or
just down from the edge of the roof.

I would advise you to also install a 2M/70CM mobile antenna on the vehicle
as well to enhance the range of the HT.  You would be surprised how many
people monitor 146.52 and who are more then happy to offer local information
and directions.
The CB can be good to use, but if there is high traffic, heavy skip
conditions or idiots making noise, they are useless for local
communications.
You can almost always find a repeater or simplex frequency with a ham on it
who will give you directions.
If you don't live in a high tourist area, you likely won't hear much in the
way of that sort of info being passed on the ham freqs in your area, but I
know that in my area, many many hams pass through on holidays and during
summer months, we usually have a couple a day come on 146.52 looking for
directions or local attractions.
If you just have the handheld rubber duck antenna, you probably won't find
people on 52 either, so the 2M mobile antenna is a must have.

hope this helps a bit.
If you are looking for a quick and easy simple but effective installation,
try one of the firestick low profile antennas as well, they are a quarter
wave helically wound antenna and perform rather well for local
communications...don't expect to work any DX with it though hi hi.  I think
you can get them around 2 feet long or so.
73
Colin, CF6BKX