Water in gas tank or something else?

[1979 GMC Sierra]

Hey guys, lately i've been having problems with my 76 3/4 ton. I will be driving on the hwy for half hour to an hour and then she starts surging and losing power and speed and then stalls. I pull over and try to start and she wont start. I wait a minute or two and try to start again and she starts and runs for a few seconds then dies. I repeat this process over and over until she stays on....then i try to drive and as soon as she gets going she surges and stalls again. I repeat this process over and over again until i make it home or have it towed (had it towed 3 times now in the past 2 weeks).

I thought there might be water in my gas tank, so i've been buying water eliminators/deicers that you pour into your gas tank, as well as pouring gas line system cleaners into the gas tank. But after 2 weeks of this, i'm wondering if there's something else wrong?

I'm about to siphon my gas tank to see if there's water in it, but before i do, any ideas of other possible problems i could be having? Thanks in advance.