Electrical – RV sub panel on main feed

Yes, you can. The systematic computer programmer in me even thinks it's a good idea, allowing a "star" topology for electrical distribution.

1. You will need 4 wires, as others have mentioned.

2. Keep the ground and neutral separate in the subpanels. The only place they should be bonded is in the main panel.

3. You will need ground rods at the new location. Yes, it's redundant, but it's still required.

4. The wire feeding the new subpanel must be protected from overcurrent by an appropriately-sized breaker. It can be at either end - in the old subpanel or in the new subpanel - or at both ends. It's up to you, but if only using one, I slightly prefer the upstream location, so that I can easily kill power to the feeder wire. If using both, 1 must be sized for the conductor, and the other can be that size or larger -- convenient if you buy a panel with a 100A breaker preinstalled.

5. While there are wires rated for direct burial, it's good that you're planning to use conduit. Oversize the conduit for ease of pulling wires, and minimize bends. While you have the trench open, run a second conduit for future expansion (ethernet? gas? macerator?). May as well run a water line, too, so you can have a sink.

6. Most electric kitchen stoves are installed on 240V/50A circuits. If you're installing something like that, then I suggest a 60A subpanel. That gives you plenty of headroom to install lighting, a small point-of-use electric hot water heater, and some convenience receptacles. Those will come in handy when doing maintenance in the area.

7. Aluminum wire is usually much cheaper than copper, but harder to work with. It's stiffer. You must use 1 size larger than with copper. The ends must be protected from corrosion with anitoxidant goop. The lugs must be torqued correctly. Some people think you should retorque the lugs on some schedule. Last time I bought wire, I wanted 8ga copper, but 2ga aluminum was 1/2 the price, so going with Al was worth it.

I quite agree, pulling three 4/0 Al wires is the kind of job a DIYer ends up getting stuck on and having to call an electrician just for his truck full of pulling tools.

How old is this work? If it was legal to wire ungrounded at the time, it may be legal to retrofit ground using the NEC rules allowed for that. For a retrofit, the rules are liberalized and you have a much wider set of choices for how to get it done. Make that argument to the AHJ and see what they say.

Also, why dig a wnole 'nother trench? You only need to dig out the old trench to the 45, which I assume is only a few feet from the house and shed. Then you can disconnect the wires, cut the pipe (carefully) and get the access you need to get the ground wire through. If the joints were not cemented, it'd be even easier. Really keeping an underground conduit watertight is a lost cause, it will fill up with water, that's why the wire has a THWN rating. So not cementing it does make sense. It's exactly what I'd do in that circumstance.

Also, if your wire is the obsolete AA-1350 type, just be careful with your terminations, make sure the lugs are made of aluminum (most are), use the anti-ox and torque them exactly to spec. I see no reason to spend a fortune on new AA-8000 wire, they both conduct electricity just fine, the issue was always at the terminations, and even that was mainly an issue for the tiny wires (10-12 AWG) on Cu terminations. On the other hand if the old wire is not outdoor rated, that's a different kettle of fish.