This is a topic that has proven to be a concern to me. When I was thinking of getting my Cyclone, I had read many posts on filling a HL RV and some of the processes I saw posted here had me scratching my head. My old Aljo was pretty straight forward. Put hose in and wait until the overflow port began to spit a bit of water out. That simple. On this forum I saw people saying to wait until you began to see water drip out from under the trailer and then you knew that the tanks were full. What??
My first fill, the gage showed "full" in no time at all. Not knowing how much had been in it to begin with, I took this as gospel and stopped filling. By the end of our second day, the gage was showing empty. It was a 5 day shake down cruise so we were without water for 3 days. Thank goodness the site had public showers.
Here is the important part... When I got back, I began to fill my tanks. I waited and waited and true to form, I began to see water drip out from the under belly. I could see holes drilled in the liner right up the center so I thought, "OK, the overfill tube must be up there and this is what other posters have said". I kept it going a bit until the dripping began to accelerate and then stopped. Seemed crazy to me but what could I do without being able to see the tanks? I was then scheduled to take the RV back in for warranty work. When I began to move out from my parking and the trailer hit the down hill portion of my driveway, a sudden flood poured out from the trailer. An amazing amount of water!! I added this to the items for them to look at and this is what was reported.
The transfer hose was attached, "as an after thought". Not properly clamped or sealed and considering the amount of posts I've seen here, it's clear the HL has an assembly problem. Secondly, the gage is not something you can rely on due to the small transfer line from tank to tank. They had to replace the plywood under the chassis due to rot and I now have a huge cut in my liner like a C-section on my "brand new" trailer. I know from personal experience that the black tape they used to seal it will let go soon and I'll be dealing with essentially an air scoop on the under belly. The whole cover should have been removed, not cut in half. They had the dang thing for 2 months almost, so there was time.
I have identified the overflow tubes for both tanks but I've yet to try and see if it all works like it should. The size of the overflow tubes are under sized like the drain line so I'll see how that goes. Over all, I feel it's a poor design and a poor assembly. If I'm the "one off", then good for all of you. A friends TH has a hugh gate like a grey tank gate to drain his fresh. The dinky little drain line with a petcock takes FOREVER to drain your tanks and apparently you can strain your tanks before the tiny overflow lines show that you have filled both. The tanks need a larger or two transfer lines to handle the water flow. As is, you will show a "full" reading prior to being half full. i.e. both tanks. Not just one.
My long winded .02 cents.
At what rate do you fill your tank? I've always assumed a water hose runs about 7 GPM. I have heard post on where someone damaged theirs due to over-expansion and I worry about this every time I fill it. I probably only fill around 2-3 GPM. Do you fill until it runs out the vent or do you stop when the gauge says FULL.
. This is especially true if you get side tracked and it continues to try to fill after it has reached capacity. 
