I wouldn't worry about the angle since most dumping stations are fairly level. Plus, you really have no practical choice but to pull up on the side your dump valves are on (usually driver's side). Black & gray tanks usually converge from opposite ends (favoring one or the other, but not both). I guess I'd favor slope away from the black tank as the most preferable. Look underneath to find which way it drains to the valve.
But, just use the common procedures such as
- Don't put items (other than septic-safe paper) down your toilet.
- Use a suitable chemical in your black tank as well as add a gallon or so of water after dumping so your tank never gets dry.
- Try and dump when your tanks are at least half full (preferably more) to give you sufficient "whoosh" when dumping.
- Driving around just prior to dumping helps loosen the poo.
- Dump black tank first, close valve, then gray tank to flush out your dump line.
- Make sure your valves are closed, lines capped, hose stored, and compartments are latched shut.
- Oh, and keep disposable gloves handy you can wear during the process.
If you're really fastidious, and your rv has a santi-flush feature, you can give your black tank an additional flush with pressurized water (using a non-potable garden hose hooked up to a water source). But read up very well before using this feature first. You must have you black dump valve open to release the water as fast as it comes in! If you have a macerator pump, it must also be on. You don't want a nasty accident.
I would just conclude with, don't over-think it. You'll probably find that it's as simple as dump when there is sufficient liquids in the tanks and always black first, shut that valve, and pull your gray tank valve last, then close and store your hose.