Is it slow in cranking, like the starter doesn't have enough power to get it over the compression stroke?
Assuming it's the usual 4K RV generator (micro Quiet or whatever)....
If the battery voltage is up and the connections are tight, the problem is most likely either the solenoid or the starter motor--my money would be on the latter. The solenoid is pretty simple to test in this case. Measure the voltage drop across the big terminals while cranking the generator; it should be zero or very very close to zero volts.
If its the starter motor, you'll have to drop the generator and remove the outside cover to get at it. Once dropped it's simple; the starter is on the top back side of the engine and held on by a couple of bolts.
The starter is a permanent magnet motor with four brushes. It probably has one brush that isn't making contact or is disconnected or otherwise is not doing what it should. With one brush disconnected, you get only partial power from the starter. (It's also possible that there are one or more burnt out coils on the armature, or both problems, or some mechanical/bearing problem, or a bunch of other things.) If it's a brush problem, it should be reasonably straightforward to repair; the brushes and holders are attached to the back lid of the starter, and that's removed by undoing a couple of nuts. Be prepared for the brush springs to go sproing and stuff to scatter when you take it off.
If you need a new starter, they're not all that expensive--$80ish for OEM units, and $40ish for aftermarket equivalents. The difficult part is getting access to the starter. (If you have to drop it, I suggest running that side of the RV up on ramps to get a couple more inches of clearance if it will be tight to slide it out once lowered.)
I've been there, done that, and didn't get the t-shirt. My generator cranks and starts like a champ now. I hope I can leave it attached where it is for a very long time to come.
