How do i solve the drift problem with my front suspension

If the alignment keeps drifting out there is an issue with the suspension. Have the suspension bushes, ball joints and wheel bearings checked. If any of these are worn it can throw out your alignment.
Are you getting any other issues? Things like vague steering, grumbling noises as you accelerate, vibration through the pedals or steering wheel, 'clonks' when hitting potholes? These are all clues to what the issue is.

instead of you fixing what probably is not broke , take it to a front end specialist for a proper diagnosis of the problem
to start with , having replace only one side is a problem in itself as that will alter the rear end alignment
after that is done , get a front end alignment
good practice is what ever is wrong on one side will be the same on the other side

A few things could do this

- underinflated tyres (causes soft sidewalls)
- badly adjusted wheel alignment (insufficient toe-in)
- worn tie rod ends
- loose steering rack end ball joints
- loose lug nuts
- loose front wheel bearings

After checking the first, this is safety critical, so take it to a front suspension mech. Mercedes are not complicated underneath, so you don't need a dealer unless you have a warranty.
'

have the front end steering joints and suspension checked by an accredited front end specialist. Check the wheel bearings and cv joints for serviceability. Although you say it is only occasionally it is a critical issue as on any occasion it may place you or others in harms way.

I had the same issue with my lincoln LS. You may need an alignment. Many shops off the service and the work is usually an hour or so. This will help with drift feelings and the wondering you mentioned.

Good luck and be safe

two possible problems...
1.) front end out of alignment
2.) brakes are dragging

Have your front end suspension and alignment inspected.

Lift the front of the pickup and try to shake the wheel. If it moves then a part (a bearing, a bolt or a pivot) of the front suspension is loose or worn out. 
If you can't feel it moving tell to another person to move the steering wheel while you hold the wheel. If the movement isn't exact and immediate the problem is in the steering box. For this specific tools are required - it's better to take it to a repair shop, you definitely wouldn't want to gamble with your safety

if not the wheels for sure suspension get an allignment

first make sure that your tires are properly inflated.

if that doesn't fix it, it could be the tires and the way to tell is to swap the front tires and if the pull changes direction, it's the tires.

If the pull doesn't change direction, it's alignment or a damaged/bent suspension component.