I have no experience with motorhomes, but on a regular vehicle (like a pickup), this could be a driveshaft U-joint or center carrier bearing (if used on your vehicle). A bad U-joint or carrier bearing will vibrate at a specific speed, often between 35 and 45 mph, especially under load (accelerating or decelerating). Also, transitions between accelerating and decelerating will be accompanied by a loud "clunk".
This is easy to troubleshoot; and you can do it yourself, or have it checked for free, or a very minimal charge. Usually, the parking brake is set, then someone observes the U-joints while the transmission is changed from drive to reverse several times. If a U-joint is bad, you can see and hear a big clunk when the driveline torque is reversed. (It will be really obvious, if the roller bearings have failed and have ground up and fallen out.) A bad carrier bearing (if so equipped) is less obvious; and you may need to drop one end of the driveshaft to check it.
If this is the problem, repair is relatively simple; although, perhaps not easy. The driveshaft must be removed, and it is heavy. Please, let us know if the U-joints are bad; and we can provide more details, if you wish to perform the repairs yourself. Otherwise, observe the repairs being done; and you can do this yourself, next time, if you wish.
There are other things that can cause vibrations. However, if this started suddenly and there were no vibrations, previously, this is a common failure.
If the U-joints and carrier bearing are OK, you might want to check to make sure all lug nuts are tight, wheels are not bent, and no wheel weights are missing.