The answer to this one is simple... Overall cord/cable too small gauge, or too long, or both. When it comes to power to your RV, voltage drop is one of your worst enemies.
Assuming you only want 15 amps to your 5th, you need a minimum #14 gauge extension cord. Some are only #16 gauge.
Too minimize your voltage drop, you may need a #12 gauge ext. cord. The wiring wiring inside your house for your 15 amp receptacle will be #14 gauge. You need to consider the overall wiring run length from the panel in your house to the panel in your RV. You could easily end up with a 100' run overall or sometimes, even much longer. I just wired up a 30A outlet in our carport with a total run of about 130' from the panel and I used #8 wire. Copper sure ain't cheap nowadays either!
You can calculate your voltage drop using the calculator on this site (via a link posted by Herk). For example, if you had an overall run of #14 100' long, with a 15 amp load, you would have an 8.1% drop, which is too high by NEC. If you had a #16 gauge extension cord 100' long, you would have a terrible voltage drop. If you have mixed gauges (#14 + #16, eg.) in your run, you'll have to do a detailed calc. by hand, unless someone knows of a calulator online somewhere.
Don't forget, you are already going to have a 25' 30A power cord out of your 5th which will contibute somewhat to the voltage drop. In your case, you *could* have a section of #10, #16 and #14 gauge wire to consider.
Don't overlook to make sure that the plugs and connectors on your cords (and wall recept.) are clean aand free of pitting. This can also add resistance.
Gil & Deb & Dougal the Springer Spaniel