A number of people have wondered if they need to carry their own air compressor so they can maintain the pressure in their tires. While keeping the correct level of pressure in your tires is important for the safe operation of your RV, I believe there are a few preventative measures that can be takes so you can avoid the need to carry around a compressor.
There are a number of things to consider before you go out and spend $20, $100 or even $300 when it may not be necessary to have a compressor for normal inflation maintenance.
First, lets lay out a plan on how to manage inflation without owning a compressor. We are going to assume you are not trying to inflate a tire that has gone flat due to a puncture or faulty valve. If your tire has gone flat and the tire bead detached from the rim you need not only high pressure but high air volume which a small compressor simply cannot provide. There are also safety concerns when it comes to inflating a flat tire. This task should be left to the professionals.
Since most tires will loose about 1% to 3% air pressure every month due to permeation through the rubber and you might also see a pressure loss of 1% to 3% due to a significant drop in ambient air temperature, there is the potential of needing to replace 5% to 6% of your air every now and then. This translates to a pressure loss of 2 to 7psi which is not a large amount but could be enough to be under your minimum pressure if you haven't set your pressure with sufficient safety margin.
I suggest that motorhomes set their cold inflation for all tires on a given axle at the minimum needed to carry the actual load on the heaviest tire on that axle PLUS at least 10%. If you have a TPMS you will quickly learn your "normal" weekly or monthly air pressure drop so as you travel you will have advance warning of when you need to "Top-off" your tire pressure. With this knowledge you can get air when you stop for fuel. Even the largest Class-A will be able to get enough pressure at the truck stops when they fill-up and smaller Class-C or Class-B can replenish the 5% pressure loss at the regular gas station.
If you are going to be parked at one location for more than a couple of weeks I would suggest one of the last things you do before getting to the campground would be to inflate your tires to the tire sidewall max pressure. This way even after a month or two you should still have enough pressure to still be above your minimum cold inflation when you leave the campground and you should stop at the first location where you can get air to replenish the inflation to your 10% cushion.
Trailers have a different problem given that I recommend they always run the tire sidewall max. The good news for trailer owners is that they have smaller tires so even a small 12 volt compressor should be able to top-off their tires. I would just spread out the task over a few hours the day before planned travel, so as to not overheat the compressor.
If you decide you still want a mini-compressor so you can add 2 to 5 psi to your tires, I would plan on spending at least $50. One thing to consider is to get a compressor rated at least 20 psi higher than the max you need. You also need to look for "duty cycle" rating. This is the % time you can run the compressor without overheating the unit. You don't want a unit that can only run for a few minutes before it must be shut down to cool. The ultra low cost compressors I have looked at seem to get very hot. This could be because the "cooling fins" are simply molded plastic rather than heat conducting metal.
Safety note You should never exceed the max inflation rating of your wheels or valves when establishing your "cold" inflation level, Some wheels may be marked but it seems that most of you will need to contact your RV manufacturer or wheel manufacturer to get the specification, in writing. I would not accept the simple "match the tire inflation" as the answer. Wheel manufacturers have a specific number and do not depend on the tire pressure as they cannot be sure what tire is going to be mounted on their wheel.
Also I would not use basic passenger tire "snap-in" rubber valves in RV application. You can review the posts on valves for more detailed information.