I have the V10 in my 37ft Winnebago ( 22,000lbs) and we tow a 2001 Jeep Cherokee Sport (4,200lbs). Initially I did find the tranny irritating, but with a zen approach of driving slower (at about the posted speed limits), I find it adequate, even on hills. While it is not my ideal, it has been entirely reliable and easy and cheap to service. The ride and handling are good, much better certainly than my prior RVs.
The OverUnder Gear (which I considered but didn't buy) is basically an added overdrive (which has a ratio halfway between factory third and factory fourth which is the factory overdrive). I will call fourth gear 4/OD. It is bolted between the gearbox and the prop shaft. For hills, you do two things, press the gearshift button which turns off the factory 4/OD, and "simultaneously" step on the OverUnder floor button which engages the new 'three and a half' OverUnder gear. I decided against it because it didn't solve the particular problem I was trying to address (an annoying automatic downshift to third on minor hills while using cruise control), and because for the driving I do, mainly up and down the I95, there are usually slow lanes on the long grades and I slot in with the big tractor trailers.
Your rig and tow make you a fair bit lighter than mine, and I think you will find the V10 good. You have to learn the tranny, and manage your driving so that you don't find yourself in second at 3750 revs which is really really noisy, and makes you jump the first time you do it. My Modus Operandi is to turn off the Factory Overdrive when I have a hill in front of me and take it steadily up it. That way you hardly ever need second gear, and even if you do, your speed is low enough the the engine is at 3000 or so which is much nicer.
I would also point out, that if you want the GM 8.1 with the Allison 5speed tranny or some other equipment that is better than the V10 and fourspeed, then you are probably into more money, partly because those vehicles will be newer.
IMHO, if you are taking over this vehicle, you should not be paying any more than what an RV dealer will pay for it. I would first take it to a couple of RV dealers and ask for a price for them to buy it from you outright, and then find out if the family is happy with that price. I'm guessing that someone in the family will go online to RV private sale sites and find very high asking prices for similar vehicles, and expect you to equal them. But this is a standard feature of private sales sites, ------- owners want ten thousand more than the actual market price. This does not represent the actual value of the vehicle you have available to you. Conversely, you could go and buy an RV and trade this one in on it, giving the family the trade-in value.
I hope I am not interfering here, since you didn't ask this specific question, but my experience with my family and my wife's family and several other families, is that it may be difficult to take over this vehicle without rancour from someone.