There could be several reasons why you have a bigger air gap at one end of the home than the other.
First, the home is not level - which could be down to several factors:
1. It was never level in the first place!
2. The wind down legs (if fitted) have failed.
3. The supports (if fitted) have failed.
4. The concrete base (if you have one) has cracked and subsided.
5. The chassis has rusted through and dropped (unlikely but possible.)
6. The supports were placed wrongly and the chassis has bent over time.
Secondly, the skirting (you don't say whether it is brick, wood, uPVC or heaven forbid, asbestos) was poorly fitted in the first place or, if brick, may have subsided, and the home is perfectly level!
Try placing a spirit level along the skirting to check the levels, and then do the same on the bottom drip rail of your home. Even better, if you can get under your home, use the spirit level along either of the arms of the 'H' shape of the chassis where the metal meets the floor bearers to check the levels.
Hope this helps!
Chris O'Brien
Chassis Doctor Ltd
www.chassisdoc.co.uk