If I have this right for what you are trying to do is; Run a trailer on a generator AND when available run it on power that is available on site. OK First to classify this, this is the same thing that is done for homes, business and hospitals for emergencies, (only backwards). A building is run on regular power and transfered to an emergency generator when power is down.
There are two ways to go at this, First; Use a transfer switch. (This is an electrical box that is wired onto the main breaker box and the generator is wired onto it). There are two types of transfer switch boxes, manual and automatic. (The automatic box senses that the main electricity is off and transfers over to the generator, the generator senses this and starts). (The manual switch box has to be switched over manually and usually the generator is started manually.) If you search sites for emergency power, emergency generators, your going to find these and the manufacturer’s involved. Wiring is done as a normal household with the transfer switch box at the main breaker box. The main box would take the electrical connection for the site and the transfer switch box would take the generator. Alot is written on this for emergency power and generators, use these key terms and I’m sure you will find sites on this. Main downfall to this IF the transfer switch fails ALL power is dead in your trailer until you get it fixed.
The second approach would be to run dual circuits in the trailer. One breaker box for the site power, One breaker box for the generator, dual electrical plugins at each spot, one for site, one for generator. This is dual expense although it has some great advantages; First, depending on your site, you may not get all the power (amperage) you need. You will end up blowing their fuses or yours. In this instance, you can us the generator for part and site power for part. Second there is no failure point like the transfer switch and one way or another you can get power. The appliances, stoves what not would have to be plugged into the right box to work! Extra work if you don’t know when you arrive if you get site power or generator. This is flat out the more expensive option and the less common option. This option also separates generator from site so there is little problem with anything electrical crossing.
If you get a large generator from a company, their engineers will be happy to help make sure their unit works with your trailer. Since your using electricity from someone else and a generator this is more complex. The wiring run to any site should be heavy gauge and waterproof. Any rain and I would use industrial rubber gloves to plug in. Electricians more involved in wiring emergency generators will know abit on this, although they may not be familiar with commercial trailer wiring. There are different electricians and I would go after the commercial electrician. Write in and let us know if this starts you off. You can also email me. (One extra word of caution, over rate everything, breakers, wiring, generator; there will be ratings in amps for breakers, wiring size, when your looking at wiring for 20 amps, go the next size larger, Over rate everything).
As I put in my original post, people sue at the drop of a hat and if there is a problem, there will be an investigation by insurance on how this was designed and wired. I’m directing all information be checked with lic. electricians in YOUR area, since each state is the certifying authority. RJF