What's the ideal temperature range to hvlp spray?

In a "production environment" you would be spraying inside a "climate controlled" spray boot to avoid the adverse painting conditions. Since that is likely not going to be the way your going to spray your trailer you need to select "the optimum weather" (including wind speed) of when to spray.

As BigBill already pointed out the temperature and the relative humidity make all the difference in when to spray. The reason you will hear recommendations of different temperatures is because the REAL issue is the actual DEW POINT which is driven by both temperature and humidity. Any good automotive paint dealer will give you the Material Data Sheets for the paint your shooting, and it will spell out all the details. The DETAILS on these sheets are very important. Considering the large amount of labor time involved, and the high cost of materials used, you will want to actually read, and follow the directions, and not do what most of us men do and "just wing it".

If you follow the manufacturer instructions on surface preparation, surface cleanliness, spray in the allowed Dew Point range, used a quality product, you will end up with a long lasting durable, good looking finish.

You need to make sure you have an "adequate air volume" to run a spray gun. It takes a HUGE AMOUNT of CLEAN/DRY AIR volume to power a HVLP spray gun. It's likely the highest volume air tool, you will ever run off of your compressor. It's right up in the same air requirement as a DA sander. What usually occurs is you overwork the air compressor, heat up the air caused by the compression process, and you get water condensation during the hot air cooling process. This water gets into the air lines, and the water mist sprays out with the paint. It's not a good thing. IMO, the best "home use" sprayer is one of the turbo HVLP sprayers, that look like a backwards working vacuum cleaner. You need to have very good fluid control, if you end up using an "airless sprayer" applying an enamel paint. They are used more for house painting with heavy latex paints.

Ideal conditions are 75F and zero humidity. Since you won't have that, you must manipulate to get best possible conditions. As for temp, you can use a faster or slower reducer to mitigate flash time with your paint system, but humidity can be a killer. ALWAYS use a line dryer to remove moisture from your air source--you can buy disposables good for one use--and remember that if you are trying to paint in high humidity you may get 'blushing', or water condensing in your newly applied paint.
You can rent HVLP sprayers ( a complete system) from tool rental locations, but if you buy an HVLP paint gun and compressor, they will be yours to use whenever. I own several HVLP guns and have used them for auto painting as well as painting home remodeling projects and of course, my Shasta.