We've explored a number of options to give us reliable internet access. WIFI boosters sound appealing on the surface, but our (limited) experience finds that even campsites that boast of WIFI service are generally inadequate. At the end of the day, it seems a reliable cellphone signal coupled with a "hot-spot" is the way to go for decent internet.
Of course, in many campsite locations, cellphone coverage can be spotty, and especially when inside your trailer, almost unusable. We've explored some of the high-end boosters, such as WeBoost ($600+.. Yikes), and looked for better options.

Finally I settled on a Lintratek 5/4/2 band booster for our ATT&T phones (other units are available for other carriers..(Amazon, of course) You must determine which BAND your carrier uses, then choose a version that supports your phone). https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

At $149.99, it represented a significant savings over the "brand" names. Best, though, I can't say enough good things about the Lintratek support people. It seems like they were following up every few days to make sure my install was going properly, and were quick to response with advice as I completed the project.

END RESULTS: This is the most important aspect of this system, so it needs to be stated here. Once the system was installed and operational, I found that, sitting inside my trailer at our storage site (with generally very limited cell reception, even outside)... with the booster turned off, NO bars. Booster turned ON.. FIVE BARS! Certainly results will vary with environment and circumstances, but I couldn't be more pleased with price, support, and performance!

This particular unit seems to have been designed for home or residential use, but we found it performs beautifully in an RV trailer environment. It comes with the booster unit, a very low profile and compact unit than can mount on any wall in your trailer. Also, a very powerful external antenna for bringing in the signals, as well as connecting antenna cable (50 feet worth)

The only thing missing is a mast for the antenna.

I used inexpensive thin-wall 1" electrical conduit, which I attached "permanently" to our ladder supports at the rear, using Adell clamps around the mast and the supports. Then I made a 5-6 ft removable antenna mast of 3/4" conduit, which slides comfortably into the 1" stationary conduit when at the campsite.

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This did require a bit of welding, as I welded a threaded nut at the upper end of the stationary fixed mast, and used a thumb screw to secure the smaller antenna mast in place once parked. I also had to fabricate a mounting plate for the antenna itself (another welding job, captive nuts and thumb screws) so the antenna can be attached to the upper mast when on station in just a few seconds. If you don't have welding capabilities, this can easily be bolted together. The removable mast and antenna can be easily removed and stowed in the pass-through in a matter of minutes.

Next was the matter of getting the cable from the antenna to the inside of your trailer. The cable connectors are called N-type coaxial. I opted for a pair of N=N type female Bulhead Panel Mount fittings: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Just drill a small hole in the exterior of your trailer (with a slightly larger hole inside to allow access to the securing nut (best to start from inside) so it terminates in a hidden location inside, such as inside a cabinet, etc)

There is also a package of N-type screw on waterproof caps for the exterior fitting when you don't have the cable attached:

You will also need a couple of N-type 90 degree fittings so the exterior (and interior) cables fit neatly against the wall : https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

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Your actual booster unit should be mounted inside your trailer as far as possible from the exterior antenna, within reason.
When using the through-wall bulkhead fiting, this will necessitate cutting the supplied 50 foot cable into two pieces. One from exterior antenna to exterior bulkhead fitting, and the second from interior-side bulkhead fitting to the actual interior booster unit. To utilize the existing cable, you'll need two additional twist-on fittings for the ends you've cut. For this, you'll need a pair of RG^ F-type Twist-On Ciaxial Connector (the supplied cable with the antenna is apparently "F-type)

AND a pair of MWRF "F" female to N Male screw on fittings: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

For my installation on a MKS 17 I decided to mount the cellbooster unit near the floor on the cabinet wall just to the left of the door when entering:
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The exterior cable bulkhead fitting went near the aft entrance-door-side of the trailer, and then I ran the cable through the cabinetry to the booster (there are already openings through all cabinets, used for heater duct), so no drilling required.

The booster comes with a 110V "wall wart" which supplies 12V power to the booster. If you're a boondocker, it would be a trivial task to connect the booster direct to a 12V source, but we're mostly "civilized" campers, so I just connected the little plug in transformer to the 110V receptacle on the same wall. The unit doesn't come with an on-off switch, so it remains powered until you unplug it. I thought there might be times when I would want to power it down, so I replaced my single toggle interior light switch at the door cabinet with an identical dual switch unit from Amazon: Click image for larger version. 

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ID:	33188 https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1 and wired the 110V receptacle through that second switch to allow powered on/off the booster.

It may sound a bit daunting, but no more so than you'd need to do with any of the other boosters, and I estimate my total cost for the system was well under $200.