How to clean canvas on a pop up camper?

This isn’t a great time of year to do it unless you have a cover big enough to park it under while it’s up.
It’s probably been folded before it was properly dry, or stored somewhere damp.
Canvas in large bundles can get very dangerous if it’s stored wet. The inside grows mold and bacteria and the heat generated can get it pretty hot. Beeswax canvas gets very hot. In a bad case it can start smouldering and catch fire and in a large tent store that’s bad news with a lot of waxed canvas around. The whole place goes up.
That’s why in the Army when we bring the big 200lb canvas tents in after manouvres they get cleaned and then well dried before they are folded tight into their valise and stored.
That takes space, which even the Army doesn’t always have enough of.

If the canvas comes off in sections it’s an easier job. Take plenty of pictures to show which way round everything goes. Life’s easier later on then.
Clean the canvas with a hose and long-handled brush and let it dry out a bit. Bad bits can be scrubbbed with soapy water but not detergent and then hosed again. Detergent will wreck future waterproofing treatments. No-go area for tents unless you get a specialist cleaner like Nikwax Techwash which will cost a nightmare for a camper. Good for clothing and sleeping bags prior to proofing with Nikwax or Polarwax. Brilliant stuff for that, but real expensive.
Let it drain off and dry till it’s damp.
Then swamp it with water-based commercial anti-mold treatment, which is often based on copper salts or on organics. Brush it in well with a fence brush or similar, every single bit of the canvas.. Nothing missed.
Outdoor shops and garden centres sell it, loads of brands.
Then it has to properly dry. Really dry. Takes time.
If it was already proofed with beeswax you can rewax it when it’s dry using beeswax diluted with white spirit to get it thin enough to soak right in to the fibres. Brush it in well and use plenty. When it dries again it’s ready for fixing onto the trailor or storing for later which is usually best so all the sections can be fitted together after the trailor is fixed up.
If it looks like there was other proofer than wax you could use Grangers Fabsil or Thompson Waterseal painted on with a wide brush. You can buy it in 5-litre cans from outdoor shops and some big department stores with a gardens section. A fence brush or wide emulsion brush does the job fine, or use a pump-up spray bottle, which can be wasteful on a windy day and does a worse job anyway since sprayed liquids don’t soak in as well as they look.
Brushing is the best way, getting it well brushed down into the fibres, but takes longer.
It dries fairly quickly and does a good job of waterproofing on tents, awnings, backpacks,etc, in cotton or canvas, or tightly-woven nylon bags.

The trailor is a lot easier to deal with without the canvas on it. Sweep or hoover, wash with soapy water, and get it dry.
Use WD40 on a cloth to wipe metal bits. It’s a good cleaner and leaves a water-repellent film. Rubber gloves are useful for that.
Wood can get re-varnished or treated with proofer when it’s dry.Flooring can be fitted or repaired, things scraped down, and paint jobs done without risk of spoiling the canvas when it;s off and then the canvas isn’t hiding bits so a real good job can be the result.
Easy up. Keep it sweet.
Bit at a time and and then it’s not such a chore.

The camper sounds pretty good. It will likely come up like new.
But it’s a long job, bit at a time, to get it done.
I’d better say have fun but I know how you must be feeling at the thought of doing all that….keep smiling, a good job is worth doing and you’ll have a pretty nice smile when you see the finished article.
And you’ll be out camping the cosy way, not worrying if some bit or other is going to hold up if the weather turns bad.
Happy camping.