A Couple of things
First ANY load will cause a slight voltage drop. HOW slight depends on the size of the wire (Starting with the winding on the power company transformer, the wire down the pole or out of the ground to the park meter. the wire from the meter to the distribution panel and from there to your site.
Also the length of those wires
The condition of the plugs outlets and connections
And of course the size of the load.. Seeing a drop when sucking 12 amps is not uncommon HOW BIG A DROP matters Some places I've been it is scary.
Next: What worries me about your post
You mention plugging space heaters into your RV.. The odds are the outlet boxes in your Rig are Uni-box style.. In this type of outlet box instead of bending wire around a well tightened screw They push the still insulated wire down in a "Wedge" that slices through the wire and makes a KNIFE EDGE connection with the wire.. 10 amps (Space heaters are 12.5) is a lot of current to pull through that small a contact.
I smelled hot wire
Others have posted photos of even hotter (As in melted) wires
I installed some 15/20 amp outlets 12ga wire stripped back and bent around a well tightened screw.. Each of these outlets has a dedicated circuit breaker (Either 15 or 20 amp) also I added a kitchen circuit. other than the outlets (And breaker) are 15 amp type still 12ga wire bent around a screw.. Less voltage drop. NO overheating of wires.
