Solar panels are relatively expensive if you buy an "already built" unit. Harbor Freight sells a set of 3 panels producing 45 watts for about 200 dollars. 45 watts won't do a lot. They would run your laptop for a while. You could opt for LED lighting as opposed to standard 12vdc lights and that would be a big help, but that too does not come cheaply. LED bulbs average several dollars each.
Most folks on this forum who are solar advocates/users and claim to get benefit from them have 200-300 watts worth of arrays. As Desert Dew says, you can build your own arrays cheaper than you can buy them, but I'm thinking that by the time you buy all the cells and the materials to build frames and the charge regulator required to manage the output, you're still gonna be out 1000+, not to mention your labor, for a decent setup and your frames are likely to be heavier and bulkier than the pre-made units.
Real estate upon which to mount them on a 20' RV is going to be at a premium. In short, watts=square footage so the more watts you want/need, the more square feet of array you'll require. More wattage generated by more array area means more "storage" capacity and that means batteries. It does no good to have 300 watts worth of solar array if you have only a single battery. 300 watts of array in sunshine for 8 hr, generates 2400kwh and that means 200 amp-hr of charge capability for a 12v battery. Most 12v units are rated at 75-80 amp-hr, so 3 would be required just to store the available charge if they were completely flat and the plot thickens if you obey the "do not discharge past 50% of capacity" rule of thumb for extended battery life. That, then doubles your battery requirement to 6.
All of this predicated on the assumption that you wish to use your electrical goodies at night as well as in the daytime. If you would be a daytime only user, then all you really need is enough array wattage to offset the wattage requirements of your toys. Most folks use lights, TV, etc at night so you must "replace" that energy the next day as well as what you use during the daytime. Perhaps a small inverter type of generator would make more sense. They are quiet and extremely portable and can be found used, sometimes, for a significant savings over retail.
