You can't paint the inside sections of any RHS members.

Red oxide is only a primer and wont last.

Think you already know whats best to do

 As Wal said red oxide is only a basic primer, used mainly on inland structural sections. Do it once , do it properly. Not sure what's on there at the moment,if galv you can re galve but yep expensive and the older the galv the harder to remove. If the trailer is painted maybe blast and cold galv for a cheaper option. But in my experience galv is the only way to go.

Speaking from experience, my father LOVES read oxide paint. The yacht I sail on has Red Oxide on the keel as a primer, and after 19 years, the steel that is constantly underwater is still in great condition. The steelwork that sits above the water line is slightly worse, but still extremely good considering where it lives.

For a trailer, unless it is made out of angle or C section, I would recommend getting it galvanized as painting will only protect the outside of RFS or SHS frame where hot dipping will get right in the inside where the rust will start.

I built a boat trailer about 20yrs ago and it was painted with 4 coats of red oxide then 4 coats of some u beauty all weather plastic paint of which the name escapes me.
All the tubular sections had the ends welded up so no moisture could enter. Apart from the very rare occasion where the paint got chipped all was still in perfect condition when I upgraded the boat and trailer.
Sealing the tubular sections is the secret to longevity in my opinion, whether it be galvanised or painted.

 when i was in steel fabrication , engineer called for 200sq.rhs colum to be used as down pipe as well and speci required  to spray coal tar epoxy pretty sure that would work ,especially if you sealed all openings to RHS & then spray but as wallf said that was a long time ago and not sure its still in existance .

Old man loves this stuff

http://www.phoenixpaints.com.au/m_04_01.html

 i have a couple of lower frame sections with surface rust ,my first problem is getting my boat of trailer to work on eg putting her on some tyres or maybe a boatlift yard but not sure yet. second is to clean rust off and use rust converter.third is give the trailer a couple of coats of cold gal,followed by a couple of coats of silver galv all applied by brush fourth bruh on fisholene,and i hope this works,hope to do this annually providing this works,any advice on my way of thinking is appreciated no critisising cheers,might be the way to go eziliving,already have replaced the rear x bolt on member 

 I use red oxide daily on our pipe work fabrication, as others have said it is only used as a primer and will not withstand

salt water, we store all of our red oxide pipe undercover as it will start to rust within a week of getting wet.

My preference would be to galvanise, if this is out of the question seal all sections to stop water penetrating internally and have

the trailer powdercoated, powdercoaters can put your trailer into an iron phosphate bath then put on a zinc rich undercoat followed by a thick cover of powdercoat, similar to what is specified for mining.

hope this helps

 Be very careful about 'sealing' the tubes. All you need is one pin hole for the tube to fill up with moisture and corrode away.

 Like Joe amato says, if you can keep on top of it year to year you might save yourself some coin. All the exposed parts (bolts, nuts etc) on mine get a good spray of Lithium. 

 cheers for the info guys. i guess i'll go down the path of actually pricing it up to get the trailer galvanised and also making the trailer out of I beams and angle verses RHS and see where im at then. thanks again.