Is this common?

It certainly is normal to be somewhat affected by side winds (including passing semis). However, it's also entirely possible that the problem is exacerbated by poor design, poor weight distribution, or poor alignment/suspension setup or both. There's not a lot you can do about a poor design (things like having too long of a rear overhang relative to the length of the vehicle), but there may be things you can do rather easily in the other areas to improve matters.

Checking the weights involves weighing the axles separately and rearranging cargo as much as practical to even things out; in particular, having too little weight on the front axle can lead to all sorts of handling annoyances. If the fresh water tank is well aft of the rear axle, traveling with little fresh water would be one way to help. Moving stuff to forward compartments or cabinets also helps.

Having the alignment and suspension checked by a competent suspension shop--one that deals with trucks preferably--would possibly help things as well. The Ford E series chassis in particular sometimes come from the builders with insufficient caster for motorhome service; ask them to adjust that towards the high end of the specification. Other suspension upgrades may also help a lot depending on the specific situation and vehicle.

No matter what is done, it's simply not going to behave like a little car in these situations, as is only to be expected given the much larger side area of a motorhome.

It goes with the Class C Territory. That said, if the front suspension and steering parts are good, the alignment is right, loading's well distributed, and tire pressures (especially Fronts on Fords) match the front axle weight (truck scale), it'll be minimized. I've done all that on ours and it drives pretty well. Still, trucks will push it like you said.

You will notice a big difference in the severity of the "push". Some semis build a much larger "air-dam" in front of themselves while others that are more aerodynamic, slide by with a much gentler nudge. IT really will make you appreciative of how much extra power(fuel) it takes to push our cracker boxes at higher speeds. What tobydad said about moving over a bit does help it gives that big volume of air more room to get away without building as much pressure between the truck and your rig.

Quite normal; When on the interstate, keep an eye on your mirror for passing
semis and anticipate the sway but DON'T fight it, go with flow and it will pass.

Keep looking in the side mirror for an approaching truck. The faster the truck passes, the more of the push and then the pull. Do not jerk the steering wheel to correct. When I see a truck about to pass, I gently move a little to the right if possible and this cuts down on the push. I have had several drivers warn me over the CB that they are about to pass, but this is rare.

Absolutely ! Those 18 wheelers move alot of wind when doing highway speeds, and you will get "rocked". You'll get used to it......

It happens often enough that you do get used to it...perhaps even anticipate it.