Would you trust the homeless guy on the street to detail your car? If not you should not assume ANY RV dealer knows the proper way to install solar and Lithium batteries, on top of everything else this is a quickly changing industry and the best installation method from 2-3 years ago, is hopelessly outdated today.
It is bad enough when the solar power specialty shops do it, and you end up with outdated, last years components, I can only imaging what might happen with a general RV dealership.
You asked how many watts of solar should you get, the answer is it depends, how are you going to use the RV, are you going to camp off grid a lot, how many electrical appliances do you want to run, where will you camp, what climate, ....
p.s. I have 400 watts of solar panels installed 5 years ago, and just upgraded to 420 AH of LiFePo4 lithium batteries, which I installed last week to replace the lead acid. You can never have enough solar, but 400 watts will generally allow me to run the essential stuff, without needing to run the generator to recharge if there is sunny weather.
You have a least a weeks worth of reading and studying before you spend any $$ on solar panels. There is no "one size fits all" and if you don't get it right you may as well have thrown a few thousand dollars in the fire pit.
A) Don't let an RV shop install solar, get a solar shop to do it.
B) How do you camp? Off grid or mostly in campgrounds with shore power? Weekend warrior or 2 week vacation or full time living?
C) What appliances do you want to run from the battery? Furnace, Fridge, pump, lights, TV?
D) Does your RV have an inverter? How many watts? What do you run with it? Microwave, TV, Coffee maker?
E) Do you have a generator? How may watts?
F) What kind/type of converter do you currently have to charge the batteries from shore power? (make and model)
The right thing depends on how you use your RV. Some people need no solar at all, others can never have enough.
And not having enough simply means you have to either drive, run the genny or plug into shore power to keep your house battery charged up. But driving will NOT charge the lith battery to full from the alternator no matter how far you drive, unless you install one of these thingies. That will get the DC voltage from the alternator's rectified DC up high enough to charge a lith battery. And I kinda doubt the RV place will install such.
Your converter should also be lith compatible.
But the solar should be able to charge it up to full without the thingy anyway, as long as you have sun and a decent lith solar controller.
So it all depends on what you want to do. You probably don't have the need to charge to full anyway.
What works for me is doing this. I would rather have more battery than more solar. But I will add solar to it later, but I really don't seem to need it much as I want to run the genny once in a while anyway. Genny's don't like to go unused for a long time. Nor do they like short runs. IMO, it's not a big deal to have no solar at all. But there are others who have so much solar that they probably could remove their genny for most of the year.
There are so many possibilities and I cannot say what the solar needs of others are. Before solar was invented, RV's got by just fine without it.
I just want to reiterate that you really need someone that knows what they are doing installing both solar and Lithium batteries, improperly installed solar can cause roof leaks that leads to wood rot, which leads to towing what may have otherwise been a good RV to the junk yard. Improperly installed Lithium batteries can lead to electrical shorts, fires, and loss of the RV and perhaps loss of your life. Way too many people treat Lithium batteries of any type including LiFePo4 like they would treat lead acid batteries, this is wrong on several levels, one notable way is in the required fusing for safety in case of a catastrophic wiring short, this can be bad enough with lead acid, but can be way worse with Lithium due to their much lower internal resistance, as they can release an immense amount of power in an instant destroying themselves as they do so, vaporizing wires, and splashing molten metal everywhere. Main fuses like Mega Fuses with an interupting capacity of 2,000 amps or ANL fuses with an interupting capacity of 2,700 amps may be fine for Lead Acid batteries in case of a short, but for Lithium which can potentially output 15,000 amps or more for a fraction of a second you really need the expensive class T fuses rated at an interupting capacity of 20,000 - 60,000 amps.
Sure that 200 amp rated Mega Fuse will blow in case of a dead short on a LiFePo4 battery cable, but it will then instantly vaporize and likely not extinguish the arc, it will in effect be like striking an arc on an arc welder which will continue to arc for some seconds until either the BMS disconnects the load, or the battery self destructs.
In other words, don't let your typical RV shop mechanic that only knows what he has read on the side of the label install Lithium batteries in your RV.
Not particularly good, Zamp is mostly kit solar setups in a box, with semi-proprietary connections, not something most professional solar installers would use, though also not particularly bad, there are a lot worse out there, there are a LOT more details you need to ask, like how do they plan to install them. Any guy off the street that was asking take a drill/driver and screw solar panels down to the roof of an RV, the question is will they use the right kind of screws and sealant so that the penetrations they make in your roof will not leak. Will they be fastened in places where the wind loading on the panels will not rip your roof up, etc.