Had tank topped off 350 gal tank, was at 70%, they filled to 80%, gauge says keep below 90%.

Why they only fill to 80% ?

It's an expansion thing.

An RV spot near me was charging $20 to fill a 5 gallon tank. It didn't matter if it was partially empty or not it was still $20. Found a place in Plano Texas that charges $2.75 a gallon. An empty 5 gallon tank cost $13.75.

This.  Have been on propane for 4 years.  All three companies I have used for fillups say 80% max to allow for temperature induced expansion.

Liquid expands when warmed.

Tanks don't expand very well.

They fill mine to that too

Same here, 80% or so, which reminds me I'd better get it done before winter really hits.

They usually fill it to 80 around here too.  If you get a fill in the cold winter months they will cheat it a little bit and go higher.  I don't think the customer would be to happy if it blew the pressure relieve valve due to an over fill and thermal expansion. That's why they typically shoot for 80%

All Tractor Supply's also refill by the gallon

Propane needs to vaporize. It can't vaporize if it's full.

My propane supplier will fill my 500 gallon tank to 85%.

The gauge isn't considered accurate enough to use for fill level.

There's a 'spitter valve' that they open while they fill. When the tank level hits the bottom of the spitter tube and LP starts coming out the nozzle, they quit filling.
Makes me wonder what the purpose of the temperature scale on the gauge is. If you fill to the spitter in January, that's noticeably more in the tank than if you fill to the spitter in August.

Like liquid water, liquid propane is not all that compressible.

The gas above the liquid IS compressible.

It is leaving expansion space for when the tank temperature rises to ambient.

Most never ask how much was placed in the tank...

RV places are usually the most expensive places to fill.  You're paying for convenience.

I prefer to use a local fuel supplier/truck stop when I'm not filling my smaller tanks from my 500 gallon tank.  They are MUCH cheaper than other places and around half the price (with more propane) of the suppliers where you drop off your tank and get a full one.

Trailer park fills on scale.
Had 5 stone empty 20s filled for $11.75 a pop.
And Lurlene wore her halter top and fuzzy bunny slippers.

My 1000gal tank is always topped off to 80-85%

Geez, I always thought the propane company was just making sure I had to call and have it filled in Feb-Mar when prices were 2-3x what they were in September

I paid $1.29gal a gal in September.

Last winter it cost me $1.79 in Feb

What about pressure? We had a couple of tanks (20#) in a camper that wouldn't light off anymore in our camper, not even the stove. Still felt pretty full. Took them to tractor supply, they topped them off, 3.3 gallons combined were added. We were like WTF? That's all?

(20# tanks have about 4.7 gallon capacity, IIRC).

Bad regulator on the camper? Did you try them on something else. Generally, because LP is a gas compressed to a liquid it maintains a fairly contant pressure until all the liquid converts to gas on it's way out the tank, that pressure is significantly higher than the pressure you use it. I don't see how it could ever happen that your tanks became full of LP gas that wouldn't turn to vapor. Either they were contaminated with something with a much lower vapor pressure, the valves are faulty (unlikley for that to happen on multiple tanks around the same time), the refill place didn't refill properly, or the regulator on the "appliance" failed... I'm inclined to think regulator.

To eliminate the regulator try them on something else.
To eliminate the valves, try a known good tank on the camper.
To eliminate the theory that the refill place didn't actually fill them up, weight them (empty weight should be stamped on them). If the weight says they're full (empty weight plus 20 lbs give-or-take) and you tried them on another appliance, then your tanks were likely contaminated with something.

I know LP is a byproduct of refining oil into other fuel products. Is it possible that a refinery mistake put other lower vapor pressure fuels (gasoline etc) into the propane line and some found it's way into a grill tank? Seems very, very unlikely to me but not impossible.

LP and anhydrous ammonia are sometimes transported in the same tanks. A few years ago there was an incident where anhydrous was accidentally pumped into some homeowner tanks. It was a big issue because many homes use copper to plumb propane and ammonia is very corrosive to copper.

That said, you know if you have anhydrous ammonia because the smell is very distinctive compared to propane.

I'd be inclined to suspect the regulator as well unless they worked after a fill and then they were just empty.

Also, many propane places won't put more than 3-3.5 for safety reasons. Thanks to fears of product liability. If you understand propane, it's not dangerous. If you are a dumbass, then you can kill yourself with a nearly empty propane tank.

Thanks guys, after reading that, I'm inclined to suspect the regulator too.

Lol, glad my thread could be beneficial to others.

I used to do home delivery of Propane.  When it was really cold we would go above 80%. The problem is the gauge isn't always accurate.  Hell there were some places that didn't even have a working gauge

The spitter is the only way to tell when you are at or near 80%.  I remember one time there was an emergency fill for a guy who was empty.   Couldn't get there until the next morning.  His tank didn't have a spitter or gauge.

He didn't tell us that another company had been there during the night to fill him up with 400 gallons

I hooked up the hose and planned on shooting in 400 gallons.  I got to 90 and the pump started squealing.  WTF.  The MFer was completely full.

I had to take off the regulator and let the liquid shoot of the pigtail until it only let vapor out.

80 or 85 percent here depending on the delivery driver.

Best deal for me