I don't know of any "rule of thumb".
You could price out the parts that are most likely to be not working. For example, a pump can be had for around $60 or so. If the water heater is bad, you're more in the ballpark of $300-$400. Tanks (black, grey, fresh) depend on whether they can be repaired (and if you'd trust a repaired one) or if they'd have to be taken completely out and replaced.
A leaky/non-working toilet depends on whether or repair or replacement is needed. Dump valves are fairly cheap, but like with most everything: It's going to run you more for labor than parts.
You might want to call your local RV place and see what their shop rate is. It varies widely...$95/hr in Red Bay last time we were there. Almost $140 at one of the shops here in NoVa.