Hi Becca and welcome to the forum! 🙂
As my respected colleague Rick said you should be fine to use this lens on the RP body. From an engineering point of view the lens mount of the RP is the same as any other R-series camera, and they are designed to take the weight of the Canon native optics.
Again, Rick has good advice on holding the camera - for several reasons, if hand-holding any dedicated DSLR or MILC (like the R-series) the correct way to hold the unit is with the weight taken on your left hand, with the heel of the hand under the body and the fingers curling up around the lens body - as per the following:
This takes all of the lens weight off the body and there is no torque on the lens mount. The techniques above also give you three points of contact with the body - eye, and each elbow tucked into the torso - providing what is essentially a tripod effect.
Now, a lot of people show themselves shooting a dedicated camera like a cellphone, but those are very light and have very, very short focal lengths, so they are relatively easy to keep still. It is NOT a good idea for the reasons you are concerned - lens(and body) weight and leverage (torque): such cameras are inherently unstable.
Furthermore, the longer the focal range, the more any lens movement is magnified within the image, the heavier the lens and thus greater the impact - there is a good reason that the rule of thumb for minimum shutter speed is 1/focal length. Finally, holding a camera away from the body is unsupported forward flexion, which is also bad for your back, especially with a heavy lens like the one we are discussing.
If, on the other hand, you have a need for the camera plus lens mounted on a tripod, for studio or convenience, then my research indicates that Canon does not, itself, make a tripod mount; but do a search on Google for "RF 28-70 lens bracket holder" and you will come up with some 3rd party offerings for this lens, for example:
I hope this resolves your concerns!
