If you plan to plug into shore power at a campground, you would probably be far better off if you stick to ready made RV style electrical connections. RV's can have 20amp, 30amp, or 50 amp hook-up's... and the 30amp is probably the most common and is what I'd recommend. You can buy RV style 30amp power cords on ebay, the prices vary but they can be bought for around $20 and up... You really only need one with the RV style connector on the male end, with open wire leads on the other end, however...if you can't find one then any 30amp RV extension cord will work, you would just have to cut off the female end.
You should consider using an RV style power center too, these typically have enough power breakers in them for your needs aboard the bus, as well as it contains a seperate section for the 12volt fuses and the connections to tie the battery and 12v power converter together....a good power converter will also function as a battery charger for your house batteries. (Seperate house batteries and coach batteries will make sure you don't drain your coach starting system while camping).
If you go to ebay and do a couple of searches, you'll come up with a lot of items.... RV POWER CORD 30AMP is one search, and go to a power seller called Tri-State Surplus
... he's got lots of stuff, and all of it at great low prices! I keep that guy book marked and buy from him regularly...he changes inventory every so often, so go back and check often. You name it, he's got it....12v power converters (makes 12v power for lights & stuff while you're hooked up to 120vac shore power), RV water heaters, Furnaces, sinks, windows...he's got it all and you will have a hard time finding lower prices anywhere!
The reason I HIGHLY recommend using RV Specific hardware, is you will find many campgrounds will scrutinize home made equipment/conversion... the RV specific stuff means you can pull into any campground and plug right in... 30amp will provide enough power to run a roof top a/c plus lighting, tv, small appliances etc without blowing breakers!!
On edit... RV POWER CORD at Tri-State