John-
Karl is right about watching how many devices are on in this scenario. If the batteries are fully charged, you can fudge a bit, and turn on the fridge to prep for a trip; but watch it if you want to add electric-side hydronic heat for example, or turn on the microwave or something. The longer the extension cord the more the voltage drop, which can damage some component induction motors on startup (they need 120 volts but are getting only 105, and burn up from working too hard). I use a 25 ft. 12 guage from my 30 amp house RV outlet. I have a big 30amp RV cord stored in the coach for campground situations if needed, but at home I don't use it, as the 12 guage is fine. If I was going to put up guests overnight or longer in the coach by the house, I'd switch to the 30 amp extension so they'd have more household use.
In my case I have a 30 to 15 adapter at the 30 amp house outlet, then the 12 guage cord with a 15 to 30 adapter at the female end, into which plugs a 15 inch yellow 30/50 pigtail, and the coach 50 amp cord goes into that.
If you are plugging into a 15 amp outlet at home, you can skip my first adapter of course. I used to run a 14 guage cord, but after its high resistance resulted in a burned adapter once, I went out and got the much larger yellow 12 guage cord, and you should also. The shorter you can go there the better... and less spendy, too... 6, 12, or 15 foot - whichever will minimally reach for you. Bigger guage, shortest length = least resistance to easy current flow, less voltage drop, and a cool cord and adapters. Be sure your adapters are high quality as well - don't skimp on price there. Inevitably, those little black adapters are what will melt and fail. That and the nifty handles are why I splurged on a nice 50/30 yellow pigtail last summer.
In my coach, I also set my Magnum inverter/charger panel for 15 amps to control things, but you may not have that option on your rig.
Joel