I've made the trip east on 40 and west to Calif many times. There are very long distances between towns. Like the others have said......tires....tires and more tires. If you can find the bolt pattern (most likely 5 on 4 1/2) and can borrow another tire/wheel or two to take with you I would. One minimum and 2-3 is not too many in this case.
I loaned my car hauler to a friend and he took it about 800 miles round tirp and the tires were less than 6 years old and only had about 10,000 miles on them. He blew one on the trip and the others were about ready to let go when he got back. I replaced them. Right before the trip one was flat, the rubber tire stem had rotted off. I checked the others and I just had to push them and they broke off. I replaced them all with metal stems. Sorry to harp on this, but it is a real drag to have a problem and have to leave the trailer and travel a couple hundred miles round trip to get a tire or tires fixed. Had that happen helping a friend move from Wyo. to Calif. The tires on her trailer gave out in Nevada and we had a 240 mile round trip to get new ones. Like someone said hopefully since he is a trailer manufacture this is all for naught.
I agree only travel during the daylight. No fun trying to fix something in the middle of the night in the dark with semi's roaring by you 10 feet away at 75-80, what they run in New Mexico and Arizona. Had that happen also. It looks like a 5 day trip to me from PA to CA if you run in the daylight. I drive fast (75-80), but with a boat that big 60-65 would be tops.
Do you have trailer wiring now? Call ahead and make sure you are wired the same way for the same plug and save some time there.
Do you have a transmission cooler on the tow vehicle?
Road wise it is a pretty easy trip. OK City can be confusing, stay on your toes there. I would not go straight into St. Louis going west with the boat. Pretty confusing right after you get to the Missouri side of the bridge and some tight turns. Take the outer bypass route that goes around south of the city.
It is going to be a great adventure and hope you don't mind some of us harping about the tires and such. We just want to see you have a safe trip with no real drama. My number is 435-678-3094 and like I said I have friends all along that route that can fix most things.
Good luck,
Sum