As good as Por-15 is reputed to be, there are other measures that could help.
First we all must realize that unless the floor is completely removed that the tops of the c-channel frames will not be reachable for any coating to cover and seal. The rest of the frame channel will prob be less than 100% accessable also. However it is foolish not to do all possible while you have access to the area.
I would try to remove the heavy rust scale with wire brushes and elec.drill motor w/ wire cup . Do not try to brush all way to bright metal. You need a bit of rust for the acid (OSPHO) to do it's work optimally..
Then douse with OSPHO rust converter (about $24 dollars a gallon). Brush it on and spray it where you cant brush with a simple spray bottle. It is water thin and a gallon goes a long long way. Put it everywhere not just on rust areas. It still becomes a great primer even on sound metal. It should be kept out of your eyes but you will undoubtedly splash some on your hands and skin. It will need to be rinsed off you, but not immediately, and is not overly "hot" nor dangerous. Just keep a wet towel nearby while under the trailer. Let it do it's magic for a couple of days, you'll see it working immediately. (it converts iron oxide (rust) to iron phosphate). It also becomes an excellent first primer.But the conversion must be sealed within a week or so at most.
Next get everything covered with red oxide primer (oil based). I'd brush it on, spraying only where I couldnt brush. Zinc Chromate Primer (yellow) would be better but I think it has been outlawed to gen.public.
Put the red oxide on heavy but thin some of it with paint thinner and let that thin paint wick into areas such as the edges of the top of the channel that are non-accessable.
Wear a cap or stocking on head cause your gonna get some in your hair otherwise.
Many people quit here, but idealy another coat of a good enamal should be placed over the red primer after 4 or so good dry humidity days.
The key to all of this is the OSPHO. It is used and touted by many merchant marine users and much of the industrial users on the gulf coast and the oil and refinery industries.
Rustoleum without the first two steps is not the answer. However Rustoleum makes an acceptable red oxide (oil based) primer for the 2nd step. And they make of course enamels for the 3rd step.
I do not think there is ANY one-step solution to successfully fend off rusting metal, for long.
If you are timid about OSPHO there are supposedly alternatives (but I doubt their effectiveness). I have used OSPHO for years with no known harm to myself. Lowes used to sell a copy-cat called Jasco I believe. It is very similar to OSPHO, but why not go with the known brand..? anyway here is a thread for the timid. Also I would suggest a search here in the forums under Ospho.