Have a look at
http://stevewillis.com/gas/gas-course-entry-requirements there's some good information on there you're probably a category 3.
At the minimum you would probably need CCN1, CKR1, CENWAT1, CPA1, HTR1, DAH1, CONGLP1 PD RPH LAV, and HTRLP2
In my opinion they'd do better to look around for someone already qualified to employ.
Our guys that have caravan & boat LPG tickets have a lot less than that Mike.
In fact I know of guys that work in Boatyards with a lot less than that.
Perhaps Kirk could confirm??
Depends what appliances you are looking to work on. Most mobiles will have cookers so you need CKR1, boilers and water heaters will require CENWAT1, I come across a fair few warm air units in vans so DAH1, some have standard fires so HTR1, closed flued fires HTRLP2. As far as I'm aware you can't do your LPG without PD and mobile homes could be either RPH or LAV depending on construction etc. I've got to pipe up a log cabin that has been designed to be classed as a LAV to get round planning laws in a couple of weeks.
If you're looking at mobile caravans or motor caravans you're looking at probably needing HTRLP3, WATLP2, REFLP2 and also WAHLP1 if you're into boats as well.
You may be able to do CCLP1 instead of CCN1 and CONGLP1 but most training centres appear to be pushing people towards going the CCN1 and changeover route. Possibly because they don't have the required appliances on LPG.
I'm aware of people with less than that but, in my opinion, they are working out of scope.