50amp shore power outlet to 15amp "side of house" receptacle

I have 50 amp also, tweety's.com had a twist lock 90 degree that connects to the camper, I think putting less stress on the connection, also they had a 50 amp to 115 twist lock to connect to that.
Then all you need is a good extension cord.
This is what I use however as you know you cannot power everything.

You can't go wrong by putting a dedicated 50 amp receptacle for you trailer outside the house to hook into. I know you may not need the amperage provided but it will be much more convenient and safer. We use our rig when its parked in the yard and I don't regret putting mine in. Its really not all that expensive to do.

We bought a Camco 55582 adapter from walmart.Its the twist in 50 amp to 30 amp, bout 1 ft long. You could then use your 30 to 15 adapter.

http://www.campingworld.com/shopping/item/50a-to-15a-rv-pigtail-adapter/72436

Just order this 50 amp to 15 amp adapter

I would use the step down adapter and a custom made 15A extension cord.
All the materials you need are available at a H/W store.

You could get a SP to plug into the outlet you plug your laptop into but it wil provide no protection if 240V is some how run through the sytem.
But with a 30A rig that is very unlikely.

On our last 30A RV, I put those induvidual SP's on everything that was sensitive; micro, TV, fridge, etc.
Now we have a 50A rig and I have an EMS on order.

In the long run you will be better served by getting an electrician to install a larger amp receptacle and use your standard power cord or if you want, have them build a short cord just for use at home.

http://www.campingworld.com/shopping/item/15-amp-male-to-50-amp-female-pigtail-adapter/102427

This is nice as well and you won't have the problem with the unit you list of having the extension cord just hanging off the part you plug into the RV.

Twist lock would not be my choice for an upgrade.