I bought a 2006 Road Trek in 2018. It does take some skill and imagination to deal with a component that is a tight fit. Also changing one thing can require changing other related components.
I went to a compressor fridge. Refrigerators are tricky I just replaced one where height and depth were ok but the width was maybe 1/16" larger. Initially I was prepared to plane or route out the interior dimension, but it turned out a bit of Dawn soap on the fridge and side walls allowed it to fit.
Other items:
The ASME propane tank. Original was 12 gal. Replacement that had fittings in the right places which could fit in same space was 8 gal. Not a big deal as propane was not a limiting parameter, less so since changing to a compressor fridge.
The 110v control and element for the Suburban water heater, which is well supported.
The inverter (12vdc to 110vac). Original 750w Tripplite was a modified sine wave AC, was combined with the battery charger, and only served a few outlets. Ok in 2006, but modern electronics need pure sine wave. I changed to a 1500/3000w peak pure sine inverter, and also had to add a separate battery charger.The electrical equipment space (under the bed/couch) is very limited. I had to add a second automatic transfer switch as that changeover was integral to the Tripplite. I also had the inverter power the entire 110v panel, and stripped high consuming loads (like air conditioning or microwave) when on inverter.
Upgraded to a smart TV to stream TV. (See added wifi below).
Increased from 90ah lead acid coach batteries to 300ah lithium, which was largest that could fit the existing integrated battery box. No room to add a larger box. Battery cabling and fuse had to be upgraded throughout. Added Victron Battery monitor.
Added 290w of solar on the roof.
Changed the Dometic roof AC to a much better Coleman-Mach. Added a wifi enabled thermostat so I can monitor temps and turn it on before we return.
Added internet - Wifi hotspot that works stationary or rolling, router,
Replaced water pump.
Sometimes a relatively minor feature just cannot be found in aftermarket products. I had 2 overhead counter 12v lights with a 2 way switch that lit either 2 fluorescent bulbs or one dim night light bulb. Could not find anything like that. Took them down and removed the fluorescent guts and replaced with LED, and retained the dim nightlight function.
The 10 gal black tank is the most critical parameter on my rig. I built a composting toilet and replaced the thetford. Set up new one to run #1 to the black tank, and compost #2. Not worth the hassle. After one trip I went back to the thetford.