I can't believe the number of people here who say "sure, go ahead", "you will be okay", "no problem", it's done all the time".
It's not only against code, but it's stupidly unsafe on many levels.
Just remember that the next Jack-leg, wanna-be electrician, to stumble upon your money saving kludge may not know exactly why you put two wires on every connection (even if he accidentally figures out that's what you have done).
Assuming he has your same level of electrical safety knowledge, he'll probably separate the wires and think he has the makings of TWO perfectly good 30amp circuits.
Also remember, the components that you will be working with (i.e. receptacles, plugs, breakers, etc..) are not made for safe and solid connection of multiple wires of that size.
Going with the more cautious of the group, I have dropped the idea of using two wires. I'll go see how much Home Depot wants to drain my bank account for #8 wire. I know 75ft won't be cheap. Thank you for all your responses. I appreciate each one of them.
1984 Holiday Rambler Imperial
1982 Tioga Arrow Class C
I often wondered why the NATIONAL FIRE PREVENTION ASSOCIATION exists. They are the authors of the National Electrical Code. In the CODE it says you can NOT parallel wires or size 0 or less. There MUST be a reason. They just didn't grab a number and want to waste ink. Too many I THINK I CAN electricans or want a be's. 5 year apprenticeship 4 hour state test. Countless hours on the job and YOU think you can. The next post will be HELP someone hooked my camper up to 230 volts. Serves you right.