Those are great install instructions, easy to follow.
Alot of folks are switching from pellet and wood to a small gas/propane for back up.
I think the allure of packing wood and buying a ton of pellets is faiding.

Make sure propane heater draws its combustion air from the outside via a pipe
If install instructions say to leave a window open when operating this is a sure sign that it could be a problem.
Just a heads up

I have include some illustrations on HUD requirements. One does allow for the IRC. Check with local juristiction. Currently temepered glass per the HUD is only required at the locations called out per the illustrations. You could have a living room with glass to the floor and meet the HUD standards ...

Its not normal to require a dirt pad. Ask to see in some type of document or code where it is required before you give them all their money. They could of scraped the ground and made a level pad. I hope they removed any vegitation under the soil they put down or its going to sink from the vegitation...

Depending on the age of your home it may be a shear wall for wind load on the side of your home.
A give away on this is, if the botton and top plates have lots fasters through them at about 3 inches on center or tighter.

You do not need to spend extra money on a water heater that states mobile home aproved. There are no MHP (Mobile Home Police) running around your state. The only state that has such a position is Wa state. First, currently all elec water heaters meet the UL standard that was established June 14 th o...

Insulate both blower and acoil compartment.
Use 1inch high density insulating fiber board, obtain from heating and cooling contractor.
Attach with spray adhesive in a can.
Adhesive can be bought at Home Depot.

You will feel the vibration from the compressor to the metal frame into the house

Kev if you have no pitch or very slight pitch look at EPDM. It is a ruberized material that comes in sheets. They fuse the seams. Make sure they do vent the roof cavity. Shingles are warranted down to a 2/12 pitch. Some shingle mfg require a min of 2.5/12 pitch.