As I understand this, the OP is wishing to power his 5'er. temporarily, from his FIL's existing service panel. The distance from house to 5'er is 40'.
One conundrum:You shouldn't pigtail in any service through the front of the service panel for that amount of time. It's a safety issue. If you want a new 30 amp receptacle, you'll have to gain access to the top, bottom, or sides of the load center. The simplest solution is to pull wire from the bottom and locate a new 30 amp receptacle in that location. If you don't know how to do this without affecting the integrity of the load center, have an electrician do it.
Your other choice is to connect multiple 20 amp or 15 amp cords to receptacles in the FIL's house. Invariably, that will be a PITA.
For connection to the 5'er, I would choose a good 30 amp extension cord. You don't want to make this the Manhattan project for the FIL (or any one else, for that matter).