TLDR, I wouldn’t home in on a specific brand, buy based on the condition first and foremost.
Motorhomes hold their value pretty well IF they are maintained and in good condition. The depreciation is usually less steep than a car and their floor value for a working one is pretty high.
The main thing to remember is that they are all coach built on a 3rd party chassis which is a mix of a van and ALKO leisure bits bolted on the back.
The base van determines the dash, engine and gear box but outside of that, it’s all coach built. Mercedes is the ‘best’ but it’s more expensive, there isn’t usually anything wrong with Fiat or Ford. Engines are pretty anaemic across the board and expect to pay more for an automatic.
Most motorhomes use kit from 2/3 manufacturers, mainly Thetford (toilet, fridge etc), Truma or Adle for heating/cooling and Dometic (Fridges and other electronics).
Some will have more kit on than then others but you need to recognise the ‘default’ spec will be based around gas and 12v led acid batteries. It’s not exactly cutting edge stuff. High end motorhomes are only just starting to move over to lithium batteries. The flip side of this is that they are simple and easy to fix when (not if) they break.
I guess what I am trying to say is you need to look at the quality of the rest of the box because the onboard equipment will be similar almost regardless of how much you spend.
I wouldn’t focus in on a specific manufacturer, focus on the condition of the specific motorhome you are buying. Look at what extras it has like solar panels, reversing camera, aircon(£1500+ to retrofit), sat dish etc.
You also want to look at the appliances and what the fuel source is for them. For example you can get 12v compressor fridges or absorption fridges which work off 240v or gas. Hobs can be gas only or a mix of gas and 230v electric. Ovens will be gas only. If it has a microwave, that will be 230v only.
You then want to match that up with how you expect to use the motorhome.
Heating and hot water is usually gas and 230v, they can usually be used electric only, gas only or a combination of the two for extra power. It’s usually either a Truma blown air system or an Adle wet system with radiators or underfloor. Adle being found on higher end motorhomes and underfloor on really high end. Most people consider Adle to be better but has more maintenance costs it’s more complex.
Leisure vehicles tend to have a pretty ‘traditional’ styling. Something about boomers being the only demographic that can afford to buy a new one. Just bare that in mind when shopping.
You then need to decide whether you want a fully integrated motorhome with a coach built cab or a semi integrated which uses the base van cab.
They have their pros and cons, a fully integrated motorhome will have a bigger and more airy cab and may have a bed above the cab. However, they are ‘custom’, if you smash the windscreen, you could be off the road for 6 months because no one stocks windscreens for them. They are so low volume, it will probably need to be made up for your order. Because everything is coach built, getting new parts at short notice can be difficult.
A semi integrated motorhome uses the same windscreen as the base van and auto glass will have one next day. The cab is the same as the base van so you can get parts at any good parts supplier or even the main dealer.
The other thing to consider is the weight, a lot of motorhomes, particularly the larger and fully integrated ones require a C1 licence and weight over 3.5T fully loaded. Cheaper motorhomes tend to be lighter and cater to the sub 3.5T market.
‘U.K.’ built motorhomes (and caravans etc) tend to have their habitation door on the near side. European built motorhomes for the U.K. tend to have their door on the offside which is the ‘wrong side’.
All this means in practice is you’ll need to drive into the pitch rather than backing in as most campsites require you to ‘pitch’ with the habitation door facing the same direction as a U.K. built unit.
Edit: some motorhomes can tow a car/trailer, some can’t. Just factor that in when looking as I’m not sure how easy that would be to retrofit, if it can be.