Wow - I got more time now, so I'll lend some opinions but also ask for some more pictures.
If you want the powder-coat smooth finish for exterior stuff I can't help you - the POR-15 is paint-over-rust-15, it wants the surface rough so it will key in. First coat fills the pores and the second coat applied while still gummy provides the armored finish. The texture will be still there though they say the aluminum filled color has the most solids to build up heavy rust pits. I used the semi-gloss and am very happy with it, even exposed for two years now it just has a powdery surface dust on it and nothing rusting on the hitch tongue or bumper brackets, etc. Ospho or other chemical treatments are not necessary. I painted over outrigger iron that had corroded to lace and its just like the day I painted it, exposed to sun and weather every day since.
Crawling under the frame and using wire wheels and scotch-brite biscuits on a die grinder is a lot of fun. The original paint gets cleaned and top coated with rustoleum or similar where its not damaged and the active rust areas get POR-15. Health hazards for dust and fumes are present but nothing out of the usual for OTC sales to the public.
There are encyclopedic postings here on filigree corrosion - my old trailers don't suffer from it.
Large flange rivets
for the belly pans. Olympic rivets
if you have to drill out the bucked factory rivets, and various length 1/8th shank rivets for trim and such - one important point here is to get 1/8" rivets with the aluminum mandrel 'nail' part so there will be no rust spot or stains as it weathers.
On the galvanized - the sanitary holding tank contents are acidic, think it had contact with any leakage? Was it rusting from inside out or outside in? Has the metal perforated? If its from the inside out there was just a posting "inexplicable bumper design
" that discussed coatings and things. Truck bedliner coating may be a valid top coat to prevent it from happening again (will it bond to POR-15?)