If you want a cheap generator, Harbor Freight Tools (
www.HarborFreight.com) has a brand new Chicago Electric 7HP 3000 watt running/3500 watt start generator on sale for $259.00. As far as cheap goes, that's about as good as you'll find for a new generator. Keep in mind, if you are going to run this thing a lot, then you are better off buying a good quality used piece like a Honda. They have quieter OHV engines that will last for years. The cheaper Chinese stuff like Champion and the Chicago Electric are probably best suited to occasional use for shorter periods. The quality just isn't there for long duration runs and high frequency of use.
To calculate the size of the generator you need, you have to add up the wattage of all the appliances you have. (900-1100 watts for a microwave, bulb wattages, etc) You won't be running everything at once, but a good rule of thumb for a camper is add up everything you would typically run on a worst case basis. Air conditioner, lights, stereo, microwave, TV, DVD, coffee maker, etc. Add 20% to that total to compensate for start up surges. Anything with a motor will spike 20-50% ABOVE its running wattage at start up for a second or two. If your genny isn't big enough, it can trip its breaker or damage the unit.
I would go with a minimum of 3000 watts/3500 surge, and ideal would be 4000/4800 watts. The big factor is your air conditioner, as that is the big juice sucker.
Hope this helps!