Okay. I just started a Survival Fog Endless game, and I'd like to share my setup. You may be wondering how to get Survival Fog Endless. I'll explain in a sec.
Okay. How to get Survival Fog Endless? It's only available on versions for the computer, as far as I know. There's a way to access a "Limbo Page" described on the Survival Mode page of this wiki (requires Cheat Engine), which includes all the other Survival Endlesses, but I don't know much about that. I do know about save files. When you get onto Survival Endless (and other games, for that matter), the game doesn't verify that you're playing a "blah blah" level, where "blah blah" can be Day, Night, Pool, Fog, Roof, Roof Night, etc. The save files are located at different locations depending on your version.
MAC version: /Users/*username*/Library/Application Support/Popcap/Plantsvszombies/userdata
PC non-steam version: C:\\Program Files\Popcap Games\Plants vs Zombies\userdata
PC Steam version: C:\\Program Files\Steam\steamapps\common\plants vs zombies\userdata
(These may be wrong. It'll be somewhere of the sort.)
The names of the Survival Hards and Endlesses are as follows (replace # with whichever user you are. View the list of users. If you're nth on the list, replace # with n.):
Game#_6.dat: Survival Day Hard
Game#_7.dat: Survival Night Hard
Game#_8.dat: Survival Pool Hard
Game#_9.dat: Survival Fog Hard
Game#_10.dat: Survival Roof Hard
Game#_11.dat: Survival Day Endless
Game#_12.dat: Survival Night Endless
Game#_13.dat: Survival Endless
Game#_14.dat: Survival Fog Endless
Game#_15.dat: Survival Roof Endless
(Note that the Endlesses that aren't Game1_13.dat are only important if you used the Cheat Engine to access the Limbo Page.)
Now, start a Survival Hard, pick your plants, start the first set of 2 flags, then save and quit. Rename your file (for instance, for Fog Endless, rename Game1_9.dat as Game1_13.dat, and make sure to back up Game1_13.dat if you want to not permanently lose your current Survival Endless), go to Survival Endless, and you should be playing the same Survival Hard on Survival Endless, but it won't end. Then, just keep playing!
You might be wondering why I picked all those plants. I'll explain the use of each one in turn.
Cob Cannons are basically for use, one pair every 12 sec should be perfect. That's pretty much all.
The Gloom-shrooms in the back take care of Diggers and Imps.
The Twin Sunflowers are for providing you with sun.
The Winter Melons are to slow down zombies, and to damage some.
The Fume-shrooms in the outer rows are there to pile on that last bit of damage to Gigas that you need to kill them.
The Umbrella Leaves are to protect you from Catapult Zombies.
Bungee Zombies are taken care of with an Ice-shroom the instant they land, most of the time. They may get to steal a Fume-shroom, but those are cheap to replace.
The Fume-shrooms in teh inner rows greatly reduce the probability of an instant Jack-in-the-box explosion that destroys two of your Gloom-shrooms in the pool, resulting in costly replacement.
I generally bring Imitater Pumpkin instead of Imitater Cherry Bomb.
Blover clears up fog and Balloon Zombies.
Puff-shrooms and Scaredy-shrooms stall Giga-gargantuars.
Hopefully, I covered the basics of this setup. Most Pool Endless setups will work here, just keep in mind the lack of delay for explosive mushrooms, the lack of need for Coffee Beans, the lack of sun falling from the sky, and the lack of view without Blover or Planterns. There are a bunch of YouTube videos from Hitoridakeno and Tammttg that have strategies for more Survival Endlesses. Good luck!