Manchester tank manufactures DOT cylinders in 20/30/40 lb sizes both horizontal and vertical with internal float gauges that should be reasonably accurate but the tanks run in excess of $200 each vs less than $100 for a non-gauge tank. You can buy 30 lb Flame King DOT cylinders (made in Thailand) for about $50 if you need new cylinders.
To answer two different questions that have been asked, the Mopeka sensors can be bought either with the aluminum spacer feet, or the urethane "halo" rings that go on the base ring. You cannot buy the sensors alone. Currently the halo rings are on sale for half price, so I bought two of the sensors with the aluminum spacer feet (about $10 cheaper than the sensor/halo combo) and then ordered the halos that are currently on sale for half price ($7.49 each) so I have both. I have not used the trailer since installing the sensors, but have no reason to think they don't work, given the reviews over the years.
The sensors are ultrasonic and mount with strong magnets on the bottom of the tanks. The app has pre-programmed settings for 20/30/40 lb tanks or you can select a specific height of fluid level in the tank. You can install them on a ASME horizontal tank on a Class A or C, or on the horizontal DOT cylinders used in some truck campers, you just have to plug in a number for the depth of the fluid. On the bottom of an exposed tank, I would use duct tape or similar as I do not trust the magnets to hold it out in the open.
The sensors have two different update time intervals, 3.5 secs and 10.5 secs. When you install them and pair them to your phone, you will want to change the setting from the default 3.5 sec to the 10.5 sec by holding the pairing button down again for 10 secs or so. This saves on battery usage and doesn't really affect the use of the sensors. I have found they work well with my phone up to about 30 ft or so. My trailer is fiberglass, not sure if an aluminum sided trailer would have any affect on signals.
Mopeka also makes a Pro sensor that will work on 500 to 1000 gal fixed tanks, and coupled with a WIFI bridge you can monitor the gas in a tank at a rental house, vacation home, etc that is far away.
There were problems with earlier versions of the sensors which have been corrected but I didn't want to chance getting some early sensors so I ordered the sensors and halos directly from Mopeka. Very fast service and I know I got the newest product. Yes I paid a little more.
Not sure what you can do with the app without a sensor paired to it, but you could download the app and play with it.
https://mopeka.com/
'03 Ram 2500 CTD, 5.9HO six speed, PacBrake Exh Brake, std cab, long bed, Leer top and 2008 Bigfoot 25B21RB.. previously (both gone) 2008 Thor/Dutchman Freedom Spirit 180 & 2007 Winnebago View 23H Motorhome.
