Tow dolly empty - 1500 miles

I think I would look into renting.

You could strap on some cement blocks on it.
I’ve seen it done.

Having been behind some empty car dollies, I would keep a close eye on it. Most that I have seen bounced all over the road sort of like fishtailing. Obviously they are not heavy enough to affect the coach handling but I figured it probably didn't do the dolly any good. Maybe uneven tire wear and bearing stress as well as possible structural damage to the dolly.

Just fasten down anything that's loose (chains/ramps) or store it inside your MH. Bouncing down the road shouldn't hurt it.

Some manufacturers recommend against it, others say it's OK. Keep an eye on fasteners obviously, and as you said drop tire pressure to like 15psi.

My only experience towing one empty was a Uhaul dolly for ~100 miles. Holy **** was that loud! Like embarrassing, people looking at what is making all that racket loud. I'm pretty sure the tires were in the air as much as they were on the ground... Didn't drop tire pressure though since it was just an emergency rental.

If I were going 1500 miles with one, I think I'd toss a few bags of concrete on it.

If it can’t take bouncing down the road empty, I would not put a car in it.

Should be alright if you keep the speed down.