We've never found Idyllwild to be very RV convenient. Minimal to no RV parking and very limited camping opportunities. Check out that grade on route 74 up into that area from the Indio / Palm Desert side before tackling it. Same for the grade up from Interstate 10 at Banning on route 243.
A backup camping alternative in that area is Lake Hemet about 7 miles south of Idyllwild on 74. Not recommending it at all but it is a place to park overnight if you are stuck up in that area. The place is rustic with lots of permanent trailers. The spaces(on grass) are virtually unmarked in any meaningful way. You have to dump on the way out.
If on your travels you will be heading up into Big Bear Lake and Lake Arrowhead use route 38 from the Yucaipa area to get up into there. It's a big loop around to the east but the easiest grade -- and through some pretty nice scenery.
Thanks for the information so far. My next question was about driving an RV up into that area! You mentioned the grade on route 74, and it looks somewhat daunting for an RV. My uncle is looking to rent my RV, and I didn't realize it, but he wants to stay up in that area for a whole month. Plus, since he is new to RV'ing, I was worring about him driving it up from Palm Springs as that road looks pretty bad to do in an RV.
It's about a 12 mile grade up out of Palm Desert and not really that bad of a grade as western mountain roads go. It is listed as a about a 6 to 7 percent grade so in spots there might even be short stretches of 8 or so. Interstate highways are not supposed to have a grade in excess or 6 percent so that gives you an idea of about what 74 would be like. But 74 has quite a few switchback type turns and deep drop offs into canyons at places. Commercial 18 wheelers can use the road up to a maximum of 38 feet king pin to axle which indicated those turns are not all that severe. If they were the max kingpin to axle would be more like 30 feet.
Climbing a steep grade up out of what could be some pretty hot temps in the desert makes it desirable to keep the RPM up in the power band and as much as possible but not have the gas pedal floored all the time. That cooks the exhaust manifolds and engine in general so backing off the throttle as much as possible every now and then is desirable -- so long as you are not giving up to much RPM that maybe can't be recovered. It can be a delicate balancing act at times. And sometimes you just have to "cook" things.
And of course the transmission temp will rise quite a bit too pulling that or any grade. The more grades you pull the quicker one should change out their tranny fluid. But one grade would not make any difference.
Hope this gives you some approximate idea of the grade on 74. It's not a "bad" climb as such but one that can keep you busy on the way up thinking about everything.
I don't mean to imply somebody is going to ruin an engine or tranny on that grade though. There is just a bit of "art" in going up and down grades and with practice it becomes second nature.
About Idyllwild. I'd assume your uncle must be familiar with some of the good hiking and such up at Idyllwild. You can hike over to the Tram above Palm Springs for lunch or maybe take the tram down and back up. The Pacific Crest Trail goes right past Idyllwild, too.
Also, the grade over at I-10 and Banning is similar but with tighter turns and limited to 30 foot kingpin to axle for commercial trucks.