Do you mean a handrail on the exterior? Do you mean some sort of handrail inside as well or just a handle?
What handrail do you want? An extended handrail style or larger grab bar? Do you have a handrail in mind already?
There will be a stud immediately on either side of the door frame. But, when a handrail is installed, the mounting "flange" may have to be notched a little to move it closer to the door frame. Also, the screws may have to be angled slightly to get into the stud. The studs aren't all that wide, but they are there and are strong enough. You cannot mount just into the siding material. You will need to anchor a handrail or grab bar into a stud.
If you google "rv handrail" or "fifth wheel handrail" you can find some google images for ideas.
Stromberg Carlson lend-a-hand:
Personally, I don't like these from an ergonomic perspective. As you enter, you are forced to swing your arm around as you climb up. I find them awkward and uncomfortable. Very popular though.
Stromberg Carlson grab handle:

I like this brand because it has really nice soft rubber which the other brand doesn't. I like this handle and is what we have on our TT. IMO, this style is easier to reach up to and pull yourself inside.
Torklift Glowguide handrail:
I don't know anything about this. My biggest question would be how stable they are. I *think* there may something else out there that is similar.
etrailer has this stair & handrail assembly but looks heavy and awkward to set up.

When it comes to actual "handrails", there really isn't much out there for exterior use. Some have fabricated their own from scratch.
On the interior, you can install an exterior type handle or taller grab handle like the above one. Again, you may need to notch the flange and angle the screws.