Electric window problem

"drivers door' meaning the opening door on the LHS of the vehicle?

this will be a Hymer conversion issue, perhaps you will be able to temove the door trim and then, if you disconnect the actual window movement, ypu could try just operating the motor.

this will tell you if its the motor that is struggling or if the actual window itself is stiff.

The simplest thing to try first would be to spray the window channels with silicone and run the window up and down a few times, repeating the spray as required. Try to spray the silicone down the channels into the door too as this may be where the tightness is.

On a mercedes saloon car I found that :-

Before applying silicone spray slide a piece of thin card along the gap between the window and seal.

An accumulation of fine grit was hampering the mechanism and triggering the safety mechanism intended to prevent the trapping of fingers

After the easy fix above has been tried ......

I've no idea what kind of mechanism you have inside your door to lift/lower your window, but the first thing would be to remove the inner door card, then carefully remove the plastic water proof membrame. Some cars use a series of gears, levers and a geared quadrant, while others use a cable (like your bike's brake cable) over a series of pulleys. Make sure any moving bits are lubricated