Quad Bike mounted to ladder?

I probably wouldn't do it without calling Arctic Fox and asking their opinion. If I were going to try it, I would make sure it is strapped so there will be no movement/bouncing around. And better would be if you could get a hitch type of carrier attached to the frame under it to support most of the weight.

A while back a contributor here wrote, "never use the ladder for anything other than climbing to the roof". I've used that as gospel since then.

I would think about a front hitch and no use the ladder.

The 300lb rating for the ladder is most likely a 'static' rating - ie a big fat guy can climb the ladder with the camper parked. Although I would be doubtful that the manufacturer did any sort of calculations and just had Bubba go climb it to test it. What you are suggesting would be a dynamic load - a load that is there while the camper is in motion. With it hanging way off the back, if you hit a dip or speed bump a little too fast, the effective load could easily exceed 300lbs due to the g-forces and leave your ladder and bike lying in the road.

I wouldn't do it.

Ladders are not designed for dynamic weight bouncing and vibrating down the road. In addition you have a 4' high bike that will increase torque applied to the ladder.

Leave the bike at home.

And if it comes off resulting in property damage or injury will your insurance cover it?

I watch RV reviews on youtube. That ladder flexes every time someone climbs up/down. I think you'd be asking for trouble. At the least, water proof seals could break down where the ladder is attached to the RV. Don't those things come with pretty hefty rear cargo hitches? I think I'd rather fab something to that.

How about a small trailer or swivel wheel dolly?

I'm not sure what a quad bike/Vier Bike is.
125 lbs is not going to destroy the ladder.
Your camper has springs and shocks, so the "bouncing" will not be so harsh, no more than you climbing on the ladder and bouncing a little.
Is this a battery bike? Take the battery out to lower the weight.
Can you secure the bike such that the tires are touching the bumper so there is another point of contact?
You will find that forum people panic quite easily. Ask and they will tell you to never even climb on the ladder as it could break.
I have seen many ladders with multiple bikes and other junk on them.
Always a potential that the ladder breaks, but Im doubtful that the ladder will fall off on a trip all at once.
You should be fine.

I found Vier Bike's website. Don't know what model the OP has, but the dimensions of some I looked at are like 87" long by 47" wide. Small wonder it can't fit in the truck bed, and would have to say the idea he's floating to haul it would be ill advised.